Similar to #33218, build artifacts were being uploaded into a CarrierWave
temporary directory in the Rails root directory before moved to their
final destination, which could cause a copy across filesystems. This
merge request refactors the work in !11866 so that any uploader can
just override `work_dir` to change the default implementation.
Closes#33274
- Moves a duplicate `file_storage?` definition into the common
`GitlabUploader` ancestor.
- Get the `uploads` base directory from a class method rather than
hard-coding it where it's needed. This will be used in a subsequent MR
to store Uploads in the database.
- Improves the specs for uploaders.