feat: artifacts can be downloaded using their id instead of their name (#8957)

The web endpoint

`/{owner}/{repo}/actions/runs/{run_id}/artifacts/{artifact_name_or_id}`

can be used with either the artifact name used when it is uploaded or the instance wide unique number of the artifact, if it is not found. For instance:

`/root/myrepo/actions/run/3/artifacts/my_artifact_name`

or

`/root/myrepo/actions/run/3/artifacts/42`

The `upload-artifact@v4` output value `artifact-url` is built in this way and is now a valid URL to access the artifact.

Refs https://codeberg.org/forgejo/forgejo/issues/6147
Refs https://code.forgejo.org/forgejo/runner/issues/187
Refs https://code.forgejo.org/forgejo/upload-artifact/src/tag/v4#outputs

## Checklist

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### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
  - [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [x] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

<!--start release-notes-assistant-->

## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Bug fixes
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/8957): <!--number 8957 --><!--line 0 --><!--description YXJ0aWZhY3RzIGNhbiBiZSBkb3dubG9hZGVkIHVzaW5nIHRoZWlyIGlkIGluc3RlYWQgb2YgdGhlaXIgbmFtZQ==-->artifacts can be downloaded using their id instead of their name<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8957
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

tests/integration/api_actions_artifact_v4_test.go

@@ -344,6 +344,7 @@ func TestActionsArtifactV4DownloadSingle(t *testing.T) {
 	req = NewRequest(t, "GET", "/user5/repo4/actions/runs/188/artifacts/artifact-v4-download")
 	resp = MakeRequest(t, req, http.StatusOK)
 	assert.Equal(t, "bytes", resp.Header().Get("accept-ranges"))
+	assert.Contains(t, resp.Header().Get("content-disposition"), "artifact-v4-download.zip")
 	assert.Equal(t, body, resp.Body.String())
 
 	// Partial artifact download