Relocate the Bootsnap cache directory, using explicit full path.

!507 (merged) did not account for this not being a relative path, and resulted in gitlab-com/gl-infra/production#2496 (closed)

This explicitly re-implements !507 (merged) by revert of revert, but adding explicity path to the environment variable. As a result, we also update mkdir and du calls to use only the environment variables.

Relates to gitlab-com/gl-infra/delivery#916 (closed)

Relates to gitlab-com/gl-infra/production#2496 (closed)

cc @jarv