Parallelise removal of the build dir while other things are being built/installed

[kit-ty-kate]

See #6906 (comment)

[There is a performance loss] rm of the build dir are sequential and block the next install when installing fast packages to build so it's very slightly slower when the builds are too fast

[rjbou]

Discussed in dev meeting: It's a performance enhancement that we could have 👍 . We need to be careful on how to implement it.