Fix -cross arg condition for "different OS" case

[am11]

e.g. freebsd-x64 build failed with:

  /dotnet/src/runtime/src/coreclr/runtime.proj(114,5): error MSB3073: The command ""/dotnet/src/runtime/src/coreclr/build-runtime.sh" -x64 -release -portablebuild=false -keepnativesymbols -os freebsd -targetrid freebsd-x64 -cmakeargs "-DCLR_DOTNET_RID=linux-x64" -cmakeargs "-DCLR_DOTNET_HOST_PATH=/dotnet/.dotnet/dotnet" -cmakeargs "-DCDAC_BUILD_TOOL_BINARY_PATH=/dotnet/src/runtime/artifacts/bin/coreclr/freebsd.x64.Release/cdac-build-tool/cdac-build-tool.dll" -component runtime -cmakeargs "-DCLR_CMAKE_RUNTIME_CORECLR=1" -cmakeargs "-DCMAKE_PRODUCT_VERSION=10.0.0" -cmakeargs "-DCMAKE_CONTINUOUS_INTEGRATION_BUILD=false" -cmakeargs "-DCMAKE_NET_CORE_APP_CURRENT_VERSION=10.0" -cmakeargs "-DCMAKE_BUILD_LIBRARIES_CONFIGURATION=Release" -cmakeargs "-DCMAKE_BUILD_RUNTIME_CONFIGURATION=Release"" exited with code 1.

because there is no -cross.

[am11]

@jkoritzinsky, before, auto-inference wasn't working with:

$  docker run --platform linux/amd64 --rm -v/home/runner/work/dotnet-riscv/dotnet-riscv/dotnet:/dotnet -w /dotnet -e ROOTFS_DIR=/crossrootfs/x64 \
    mcr.microsoft.com/dotnet-buildtools/prereqs:azurelinux-3.0-net11.0-cross-freebsd-14-amd64 \
      ./build.sh --clean-while-building --prep -sb --os freebsd --rid freebsd-x64 \
        --arch x64  -p:OfficialBuildId=$(date +%Y%m%d).99

same command works with "different arch" like linux-riscv64 but not freebsd-x64, and according to @Thefrank, we had to additionally pass -p:PortableBuild=true -p:PortableTargetRid=freebsd-x64 -p:CrossBuild=true props. Now auto-inference is working for differ-only-by-OS case as well. Here is a freebsd-x64 run with the aforementioned command: https://github.com/am11/dotnet-riscv/actions/runs/19563657982.

@tmds, I think we have some non-portable coverage here, but if you could verify it that would be great! :)