Use net8.0 TFM when building with source-build

[mthalman]

When attempting to enable PVP flow for the command-line-api, a prebuilt was detected for Microsoft.NET.ILLink.Tasks.7.0.100-1.23062.2. This occurs as a result of the enablement of trimming for the System.CommandLine project.

This is resolved by configuring the repo to target the net8.0 TFM instead of net7.0 when building with source-build.

Let's use this PR to iterate on any requested changes. Once we finalize the changes, I'll create a patch for source-build to use in addition to merging this PR.

cc @mmitche @MichaelSimons @jonsequitur

[jonsequitur]

Why is this being removed?

[mthalman]

Because it had no impact. The property being set was identical between the two conditions. If you want it back, can you suggest how it should be defined given these current changes?

[MichaelSimons]

These changes LGTM. I do have a related question though. I am concerned that repo level prebuilt detection is not working as expected here. Why wasn't the net7.0 shared framework being flagged as a prebuilt prior to this change? Can you follow up on this or log an issue to track this down?

[mmitche]

These changes LGTM. I do have a related question though. I am concerned that repo level prebuilt detection is not working as expected here. Why wasn't the net7.0 shared framework being flagged as a prebuilt prior to this change? Can you follow up on this or log an issue to track this down?

Probably because it's using 7.0 SDK: https://github.com/dotnet/command-line-api/blob/main/global.json#L3

[MichaelSimons]

Probably because it's using 7.0 SDK: https://github.com/dotnet/command-line-api/blob/main/global.json#L3

Of course, that makes sense.

[mthalman]

@jonsequitur - I've reverted the changes to the sample projects so that they use a hard-coded net7.0 TFM. That makes more sense to allow them to be standalone projects that can be copied/reused by customers.

[mthalman]

@jonsequitur - Can you review my latest update? Go ahead and merge if it looks good.

[jonsequitur]

Thanks, @mthalman!