Tracking: Ensure the integrity of toolchains, handle concurrent reads/writes

[rami3l]

Note

The following might be relevant:

• rustup 1.27 fails when installing targets on Windows in GitHub Actions #3709
• Create toolchain directory atomically leanprover/elan#121

[djc]

I don't find the name very clear/obvious.

[rami3l]

I don't find the name very clear/obvious.

@djc Sorry, that was a typo. Thanks for pointing it out!

PS: My mind is probably not very clear today for some reason...