mtime-on-use as a config option

[Eh2406]

#6477 told Cargo to touch files that it reads even when Cargo does not use the mtime of that file. This is needed for outside tools (like cargo-sweep) to clean up the target directory. Unfortunately this broke the playground. So it was put behind the mtime-on-use feature flag in #6573. It would be easier to test as a set and forget config option which would come with a env var automatically. This setup, an unstable config option for testing a new internal detail, was used for #6883, and it worked well. So at the last Cargo meeting the Team decided it was ok to switch the mtime-on-use feature flag to a test-track-file-usage config option. This option would be well documented as unstable and still in development. It would also be ignored if Cargo is being run on the stable channel.

Help making this happen would be lovely.