Trim away netframework targets in source-build

[NikolaMilosavljevic]

Summary of the changes

This is part of the source-build work to enable trimming of net4* targets. Parent repos (installer and sdk) have been updated, so the next step is to update dependencies, like diagnostics.

Here's how this is consumed: dotnet/arcade#12785

Top-level tracking issue: dotnet/source-build#3014

This essentially just avoids building net4* TFMs when building Linux source build.

Fixes: #3341

[NikolaMilosavljevic]

One check is failing: diagnostics-public-ci (Build and Test Diagnostics Windows Build_Release)

However, my change only affects Linux source-build and the failing check is on Windows, so it is unrelated. I'm going to rerun the check just in case, but we should be able to get this merged regardless.

Source-build check was successful: diagnostics-public-ci (Build and Test Diagnostics Source-Build (Complete))