Project Template tests place build output in the src directories

[ericstj]

Description

People expect to be able to do a fast clean by deleting artifacts and rebuild. That works for most other dotnet repos.

This doesn't work for extension due to data being written and .gitignored in the ProjectTemplates

Reproduction Steps

Build extensions.
Delete artifacts
Build again.

Expected behavior

Build succeds

Actual behavior

Build fails due to missing folders under artifacts that are referenced by generated nuget.configs in ProjectTemplates.

Regression?

No

Known Workarounds

git clean the whole repo (expensive! due to redownloads)
git clean just the ProjectTemplates directory (works, but hard to discover).

Configuration

No response

Other information

It would be nice if source directories are treated as immutable. All mutable build state needs to be placed under artifacts.