Cargo should not allow crates to tread on the target directory

[aidanhs]

The tests for tarpaulin run cargo clean, which nukes the target directory.

https://github.com/xd009642/tarpaulin/blob/083fec9ef018dd38516d1929b0fbedf06042fd60/src/lib.rs#L109

[aidanhs]

@pietroalbini does run-graph fix this by not sharing the target dir? My memory fails me...

[pietroalbini]

@aidanhs it sort of reduces the impact, because there are NUM_THREADS target directories instead of one for each toolchain. The solution for this is to lock the target directory while running tests, which should not cause too much trouble (tests are built before).