hive-forge: mirror actions/upload-artifact alongside actions/checkout

With actionsFromSelf, Forgejo resolves bare `uses: actions/<name>` against
the forge itself, so every action a workflow names needs a mirror there.
Only actions/checkout was seeded, and coverage.yml's
`actions/upload-artifact@v3` has failed at job setup with "repository not
found" since the CI runner moved to swarm-ci-runner (e2da5509).

The single hardcoded checkout mirror becomes a list of action names both
share, mirrored from github.com/actions/* like checkout already was. v3
points at the same commit (ff15f03) on github.com and code.forgejo.org.

The controller's forge-objects pass creates any missing mirror on every
pass, and the changed SWARM_CONTROLLER_FORGE_MIRRORS env restarts the unit
on switch, so an existing forge picks the mirror up on deploy.

Closes #5040

docs/swarm/README.md

@@ -283,7 +283,8 @@ one per hive. It ensures them at start and every five minutes after
   whitelists = the `operators` team, and no user. The controller leaves a
   repo with no `main` rule alone;
 - the pull-mirrors from `deploy.forgejo.mirrors` on the controller's
-  host (with the `actions/checkout` one `deploy.forgejo.actionsFromSelf` adds);
+  host (with the `actions/checkout` and `actions/upload-artifact` ones
+  `deploy.forgejo.actionsFromSelf` adds);
 - `internal/docs` (private) and `internal/knowledge` (public, with a
   seed `README.md` while empty);
 - the `agent-configs` org avatar