Fix incorrect app host used in build when PublishSingleFile=true

[elinor-fung]

• Always use apphost for build
• Make creating single-file app host using singlefilehost part of publish
  • Only if single-file, self-contained, and .NET 5+ - otherwise just uses the app host created during by build

Fixes #29795

[nagilson]

Make creating single-file app host using singlefilehost part of publish

It_builds_using_regular_apphost_with_PublishSingleFile

I am confused, this test seems to validate that it does the opposite thing I think it should do? I am sorry for my misunderstanding.

I would expect the single file host to be used when the app is self contained, net 5+, and published as single file... so why does this test that it uses the regular app host and not the single file host and seem to prove that this is true?

[elinor-fung]

The issue here was that we were using the single-file host during build rather than only for publish. That resulted in poorly constructed output where it'd be a self-contained layout with the runtime in the output directory, but with the single-file host instead of the regular host. The intention of this change is to make build always use the regular app host and make publish use the single-file host (when applicable).

So this is testing that build with PublishSingleFile=true should use regular app host and not single-file host.

[nagilson]

Ah, derp -- I totally see now :) Thank you for clarifying, I should've noticed that this was build. I assume there is already a test for publish that checks that it uses a single file host?

[elinor-fung]

There's tests for publish that check the output only has the expected files (so no runtime) and that the app runs properly, which it wouldn't if it weren't the single file host. I figured that was reasonable existing validation, but I can add something to explicitly check the msbuild items if you want.

[elinor-fung]

@nagilson any other concerns / questions here?

[elinor-fung]

@dotnet-policy-service rerun

[dsplaisted]

Thanks a bunch @elinor-fung!