Use explicit filename source in config file artifact upload step of "Sync Labels" workflows

Starting from version 3.2.0 of the "actions/upload-artifact" action, "hidden" files are not uploaded by default. The
action considers a file "hidden" if any component of the path starts with `.`.

The "download" job of the "Sync Labels" workflow downloads each of the shared label configuration files from and uploads
them to GitHub Actions workflow artifacts for use by the subsequent job. Since the names of the configuration files
don't start with `.` and they aren't located in a subfolder that starts with `.`, we would not expect that this job
could be impacted by the new hidden file handling behavior. However, it was impacted after all, under certain
conditions. Previously, wildcard patterns were used in the `path` input of the job's "actions/upload-artifact" action
step. It turns out that in the case of wildcards, the entire absolute path to the file is considered in the
determination of whether it is "hidden". The "workspace" in which the workflow's steps are performed is under a path
that includes the repository name. So if the repository name starts with a `.` (e.g., `.github`), then the
"actions/upload-artifact" action step failed spuriously:

```
Run actions/upload-artifact@v3
Error: No files were found with the provided path: *.yaml
*.yml. No artifacts will be uploaded.
```

This repository does not have a name that causes this problem, but this workflow is a copy of a "template" which is
designed to be usable in any of Arduino's repositories, which might have problematic names, and so the defect had to be
fixed in the upstream file. In order to facilitate its maintenance, it is best to keep this file in sync with the
upstream.

Alternatively, this defect could have been fixed by setting the "actions/upload-artifact" action's
`include-hidden-files` input to `true`. However, it actually doesn't make sense to use a wildcard in the `path` input
when the name of the single file is already available (the wildcard approach is a vestigial remnant of a previous
version of the workflow that downloaded all configuration files in a single job, before it was changed to using a job
matrix). By changing the `path` input value to the file's explicit relative path, it is ensured that the file will never
be treated as "hidden", regardless of the repository name.

Author: per1234

.github/workflows/sync-labels.yml

@@ -75,9 +75,7 @@ jobs:
       - name: Pass configuration files to next job via workflow artifact
         uses: actions/upload-artifact@v4
         with:
-          path: |
-            *.yaml
-            *.yml
+          path: ${{ matrix.filename }}
           if-no-files-found: error
           name: ${{ env.CONFIGURATIONS_ARTIFACT_PREFIX }}${{ matrix.filename }}