FIPS build cannot run shell executor by default

As described in #418292 (closed), running the FIPS image with a shell executor fails out of the box for two reasons:

1. In the prep job it's throwing mkdir: cannot create directory '/builds': Permission denied
2. In the build jobs it's throwing Password: su: Authentication failure.

This happens because due to gitlab-runner@51d5167c:

1. The image runs as UID 1001 by default (a non-existent user). This user doesn't have the ability to mkdir /builds.
2. --user=gitlab-runner is passed to the runner. This causes the shell executor to run su: https://gitlab.com/gitlab-org/gitlab-runner/-/blob/f29584e83f2b39572d44cc15efbced87a928d1b4/shells/bash.go#L361-375. However, normally su can only be run by root, so the we get the Password: su: Authentication failure error.

@ratchade Was UID 1001 supposed to be gitlab-runner? On the FIPS image, it appears UID 998 belongs to gitlab-runner.

Perhaps we should consider modifying the shell executor to run su only if the current user doesn't match the target user.