Setting 'CreatePackage=true' for an iOS build creates a pkg

[rolfbjarne]

Setting CreatePackage=true for an iOS project:

<PropertyGroup>
    <CreatePackage>true</CreatePackage>
</PropertyGroup>

makes the build create a .pkg (in the _CreateInstaller target). This is rather useless, so we should make it so that the _CreateInstaller target only runs for desktop platforms.

And if the developer doesn't have /usr/bin/productbuild, then the build fails.