Specify sketches report artifact name in "Report Size Deltas" CI workflow

Even though the default artifact name is in use, reliance on this default is perhaps a bit more prone to issues.

Author: per1234

.github/workflows/report-size-deltas.yml

@@ -20,3 +20,6 @@ jobs:
     steps:
       - name: Comment size deltas reports to PRs
         uses: arduino/report-size-deltas@main
+        with:
+          # The name of the workflow artifact created by the sketch compilation workflow
+          sketches-reports-source: sketches-reports