ci: Save Workhorse's WORKHORSE_TREE file to avoid re-building it

What does this MR do?

Due to a combination of !49014 (merged) (which made sure we save only the binary files as artifacts) and 4d03a5f1 (which introduced tmp/tests/gitlab-workhorse/WORKHORSE_TREE to avoid rebuilding workhorse), we need to also save the tmp/tests/gitlab-workhorse/WORKHORSE_TREE file otherwise Workhorse will rebuild the binaries.

• Before: https://gitlab.com/gitlab-org/gitlab/-/jobs/1099528690#L222
• After: https://gitlab.com/gitlab-org/gitlab/-/jobs/1102812276 (no Setting up GitLab Workhorse)

Does this MR meet the acceptance criteria?

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Related to #324415 (closed)