Restore the use of environment variables for controlling the active mutation

Restore and improve the use of an environment variable to control the active mutation. Now each runner gets a specific environment variables to prevent race condition when executing the test. The variable is used when there is a single active mutation for the test session.

Also, disable coverage analysis based optimizations when coverage capture seems to have failed (no coverage reported).

This changes will allow support for xUnitV3 tests, albeit slow.

Note: I had to upgrade the F# integration test project to Net 8.0 because Net 6.0 was not available on the CI runner and this broke the build. I see no reason why this could be related to the changes I made, so I assume this is due to Net 6.0 being phased out by MS.

stryker-mutator / stryker-net

Restore the use of environment variables for controlling the active mutation #3122

Quality Gate passed