Fix Alpine SB legs

[mthalman]

The changes from #19222 caused a regression for source build Alpine build legs, causing the following error:

/vmr/src/runtime/src/coreclr/tools/aot/ILCompiler/ILCompiler.csproj : error NU1100: Unable to resolve 'runtime.linux-musl-x64.Microsoft.DotNet.ILCompiler (>= 9.0.0-preview.4.24218.7)' for 'net9.0'. PackageSourceMapping is enabled, the following source(s) were not considered: prebuilt, previously-source-built, reference-packages, source-built-command-line-api, source-built-emsdk, source-built-source-build-externals, source-built-transport-arcade, source-built-transport-cecil, source-built-transport-emsdk. [/vmr/src/runtime/Build.proj]
/vmr/src/runtime/src/coreclr/tools/aot/ILCompiler/ILCompiler.csproj : error NU1100: Unable to resolve 'runtime.linux-musl-x64.Microsoft.DotNet.ILCompiler (>= 9.0.0-preview.4.24218.7)' for 'net9.0/linux-musl-x64'. PackageSourceMapping is enabled, the following source(s) were not considered: prebuilt, previously-source-built, reference-packages, source-built-command-line-api, source-built-emsdk, source-built-source-build-externals, source-built-transport-arcade, source-built-transport-cecil, source-built-transport-emsdk. [/vmr/src/runtime/Build.proj]
/vmr/src/runtime/src/coreclr/tools/aot/ILCompiler/ILCompiler.csproj : error NU1100: Unable to resolve 'Microsoft.NETCore.App.Runtime.linux-musl-x64 (= 9.0.0-preview.4.24218.7)' for 'net9.0'. PackageSourceMapping is enabled, the following source(s) were not considered: prebuilt, previously-source-built, reference-packages, source-built-command-line-api, source-built-emsdk, source-built-source-build-externals, source-built-transport-arcade, source-built-transport-cecil, source-built-transport-emsdk. [/vmr/src/runtime/Build.proj]
/vmr/src/runtime/src/coreclr/tools/aot/ILCompiler/ILCompiler.csproj : error NU1100: Unable to resolve 'Microsoft.NETCore.App.Host.linux-musl-x64 (= 9.0.0-preview.4.24218.7)' for 'net9.0'. PackageSourceMapping is enabled, the following source(s) were not considered: prebuilt, previously-source-built, reference-packages, source-built-command-line-api, source-built-emsdk, source-built-source-build-externals, source-built-transport-arcade, source-built-transport-cecil, source-built-transport-emsdk. [/vmr/src/runtime/Build.proj]

This is due to the change made in dotnet/dotnet@546a6bb#diff-e98dbe2aa8231e9d6264210a63f02e97b5e26de2cca16db7ee471a2257f32c8f to define targetOS for the Alpine build legs. That change was intended to make this check succeed: https://github.com/dotnet/dotnet/blob/00d6710677d75715dedef053b70a34b09af8bc47/test/tests.proj#L14.

The fix is to remove the setting of targetOS and update the test condition to not apply for source build since source build doesn't use portable RIDs.

[MichaelSimons]

Can someone help me understand why __PortableTargetOS is initialized to linux-musl on alpine while targetOS is linux? I'm trying to understand if the PortableTargetOS comparison against TargetOS is correct.

Given this is blocking. We should merge and circle back if necessary.

[mthalman]

Are you asking in the context of SB?

[MichaelSimons]

Are you asking in the context of SB?

No, in general.

[mthalman]

Let's look at an example. In UB cross build scenarios of building Alpine from Mariner, __PortableTargetOS is set to linux here: https://github.com/dotnet/arcade/blob/be933308b9024d798a9a22c0b8f3c8e3616ffbd8/eng/common/native/init-distro-rid.sh#L111

And TargetOS is set to linux-musl as a build property here:

installer/eng/pipelines/templates/jobs/vmr-build.yml

Line 327 in d092a8a

extraBuildProperties="$extraBuildProperties /p:TargetOS=${{ parameters.targetOS }}"

This build property doesn't impact the value of targetOS in the init script that __PortableTargetOS is derived from.

/cc @ViktorHofer

[ViktorHofer]

I'm not sure if we ever set DotNetBuildSourceOnly to false. This should probably be changed to '$(DotNetBuildSourceOnly)' != 'true'".