Put mailroom binary in the correct location on UBI

What does this MR do?

Put mailroom binary in the correct location on UBI

- Ask UBI to install mailroom to the correct system path rather than
  inside an assets subdirectory.

Closes https://gitlab.com/gitlab-org/build/CNG/-/issues/2172

Signed-off-by: Robert Marshall <rmarshall@gitlab.com>

Related issues

Closes Mailroom UBI container incorrect pathing /entry... (#2172 - closed) • Jason Plum • 17.11

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated
• New dependencies are managed with GitLab forked renovatebot