Fix SDK resolution in MicroBuild Plugin installation

[ellahathaway]

Builds that specify an SDK version in their global.json are experiencing failures when trying to install the MicroBuild plugin:

Starting: Install MicroBuild plugin (Windows)
==============================================================================
Task         : MicroBuild Signing Plugin
Description  : Installs and configures the MicroBuild signing plugin for use during the build
Version      : 4.101.166
Author       : Microsoft
Help         : This step should be added before the MSBuild/Visual Studio Build step
==============================================================================
signType: real
zipSources: false
version: latest
feedSource: https://dnceng.pkgs.visualstudio.com/_packaging/MicroBuildToolset/nuget/v3/index.json
skipAssetTest: false
ConnectedPMEServiceName: c24de2a5-cc7a-493d-95e4-8e5ff5cad2bc
Configuring the Plugins folder: /Users/runner/work/_temp/MicroBuild/MicroBuild/Plugins
Attempting to download MicroBuild.Plugins.Signing
/Users/runner/work/_temp/.dotnet-microbuild/dotnet nuget add source https://dnceng.pkgs.visualstudio.com/_packaging/MicroBuildToolset/nuget/v3/index.json --name internal 1309 Feed --configfile /Users/runner/work/_temp/MicroBuild/MicroBuild/Plugins/nuget.config --username ado --password *** --store-password-in-clear-text
The command could not be loaded, possibly because:
  * You intended to execute a .NET application:
      The application 'nuget' does not exist.
  * You intended to execute a .NET SDK command:
      A compatible .NET SDK was not found.

Requested SDK version: 10.0.100-rc.2.25502.107
global.json file: /Users/runner/work/1/s/global.json

Installed SDKs:

Install the [10.0.100-rc.2.25502.107] .NET SDK or update [/Users/runner/work/1/s/global.json] to match an installed SDK.

Learn about SDK resolution:
https://aka.ms/dotnet/sdk-not-found
8.0.415 [/Users/runner/work/_temp/.dotnet-microbuild/sdk]

##[error]Error: The process '/Users/runner/work/_temp/.dotnet-microbuild/dotnet' failed with exit code 145
Nuget.config authenticated successfully
Creating temporary restore project: /Users/runner/work/_temp/MicroBuild/MicroBuild.csproj
/Users/runner/work/_temp/.dotnet-microbuild/dotnet new console --force --no-restore --output /Users/runner/work/_temp/MicroBuild
The command could not be loaded, possibly because:
  * You intended to execute a .NET application:
      The application 'new' does not exist.
  * You intended to execute a .NET SDK command:
      A compatible .NET SDK was not found.

Requested SDK version: 10.0.100-rc.2.25502.107
global.json file: /Users/runner/work/1/s/global.json

Installed SDKs:

Install the [10.0.100-rc.2.25502.107] .NET SDK or update [/Users/runner/work/1/s/global.json] to match an installed SDK.

Learn about SDK resolution:
https://aka.ms/dotnet/sdk-not-found

Per #16091 (comment), the following steps need to be addressed to resolve the failures:

• Allow the MicroBuild plugin installation to specify CWD for .NET SDK resolution: https://dev.azure.com/devdiv/DevDiv/_workitems/edit/2618774
• Emit a global.json into the temp directory where the MB install happens

[ellahathaway]

cc @eerhardt

[ellahathaway]

@mmitche - As part of this work, do you know if any changes need to be made to how MicroBuild invokes dotnet during signing? Would they be picking up the root global.json there too?

[ellahathaway]

Also, MicroBuild told me that the only component that needs .NET 8 that they're aware of has roll-forward enabled now, so I wonder if just using the .NET version specified in the root global.json will work? Then we wouldn't need the CWD change and temp global.json. My main concern with relying on the root global.json is that repos would be using different versions of .NET for signing. That means if something breaks, it could be difficult to diagnose why.

[mmitche]

Hmm...thinking through things:

• There could be cases where the root global.json doesn't have an sdk element. In which case we need to install one to be able to do the initial nuget restore.
• If the root global.json does have one, we could use UseDotNet@2 and tell it look at the global.json, but if we are ever using a non-official version (often), then it'll fail.
• Not too concerned about teams using different versions of the SDK for signing itself.

The roll forward is very useful though:

• I think that at signing time, we could definitely use the arcade-restored version, no problem at all.
• That just leaves the need to have an SDK available for the NuGet package restore and plugin install. Which puts us right back into the place where the global.json might reference an unavailable SDK.

[ellahathaway]

That just leaves the need to have an SDK available for the NuGet package restore and plugin install. Which puts us right back into the place where the global.json might reference an unavailable SDK.

Thanks, Matt. Sounds like we're still going to need the CWD change from MicroBuild then, but it's only needed for the plugin installation step. @mattnorflus - can you please help with the CWD change? Thanks in advance.