[7.0] SDK content test for Alpine is not comparing against linux-musl-x64 version of MS SDK tarball

[mthalman]

The SDK content test, when running in the Alpine build leg for 7.0, ends up comparing the linux-x64 version of the MS SDK tarball instead of the linux-musl-x64 version. This is not an accurate test. Since it's Alpine, it should be checking the content of the linux-musl-x64 version to ensure that content matches correctly.

This happens because the PortableRid MSBuild property is set to linux-x64 when it should be set to `linux-musl-x64. This is set correctly in the 6.0 branch:

https://github.com/dotnet/installer/blob/1b5e0e204956294804c290739a41e440d184d0af/src/SourceBuild/tarball/content/Directory.Build.props#L184

This was discovered while testing the changes in dotnet/installer#13074. Because PortableRid is set correctly there, it ends up using the linux-musl-x64 version of the tarball and exposed another unrelated issue.

Related to dotnet/installer#17324.

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.

[MichaelSimons]

[Triage] Please investigate if this applies to 8.0 as well when fixing.

[ellahathaway]

8.0 does not currently include an alpine leg for the SDK content tests. However, the regex pattern for downloading the MSFT sdk matches that of 7.0 (i.e. uses linux instead of linux*). Therefore, in order to address #3715, this pattern will have to change to be linux*.