TargetRid default needs to be updated

[tmds]

Currently source-build determines the non-portable rid of the target platform by calling RuntimeInformation.RuntimeIdentifier:

<TargetRid Condition="'$(TargetRid)' == ''">$([System.Runtime.InteropServices.RuntimeInformation]::RuntimeIdentifier)</TargetRid>

The changes in dotnet/runtime#89598 mean this property will no longer return the rid based on Linux's /etc/os-release, but instead it will return the rid the .NET runtime was built for.

So when source-building .NET with a portable Microsoft SDK, the RuntimeIdentifier will be the portable rid of that SDK, and not the non-portable rid of the platform the build is running on.

The TargetRid should continue to default to the non-portable rid of the platform so we can identify portable from non-portable assets.

cc @MichaelSimons @elinor-fung @dsplaisted @omajid

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[dsplaisted]

Linking this to the change where this occurred: dotnet/runtime#89598

[elinor-fung]

Runtime (and some other repos) use init-distro-rid.sh (and init-os-and-arch.sh). Unfortunately, that is not in a common location - other repos just take syncs from dotnet/runtime. But I think it would make sense to have https://github.com/dotnet/installer/blob/main/src/SourceBuild/content/build.sh use that too (maybe a copy until we can get it into arcade), so it would get the same $(__DistroRid).