Implement XDG Directory for workspace base image
MR: Pending
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## Description
Follow-up on discussion here, https://gitlab.com/gitlab-org/gitlab-build-images/-/merge_requests/983#note_2618725341, we would like to implement XDG Directory, so the user would have post-start script result perisitent on volume.
But we should first wait until the issue for renaming `/project` to `/workspace` (issue https://gitlab.com/gitlab-org/gitlab/-/issues/553096+), and Persist desired kubernetes configuration on workspace creation+ to be rolled out first.
The original MR should have got some implementation in place already for reference, https://gitlab.com/gitlab-org/gitlab-build-images/-/merge_requests/983_
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## Acceptance criteria
- [ ] implement XDG directories to use `/workspace`
- [ ] Test behaviour works as expected (for example, `mise install` in post-start script will presistent)
## Implementation plan
NA
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issue