allow bypassing the build directory lock As bootstrap locks its entire build directory, parallel bootstrapping for anything becomes impossible. This change enables developers to bypass the locking mechanism (with `--bypass-bootstrap-lock` flag) when it is unnecessary for their specific use case. more context: https://rust-lang.zulipchat.com/#narrow/stream/326414-t-infra.2Fbootstrap/topic/Build.20.28miri.3F.29.20sysroots.20in.20parallel cc `@saethlin`