feat: use download-artifact instead of manually downloading it

This should hopefully work on private repos too!

.forgejo/workflows/check.yml

@@ -33,6 +33,8 @@ jobs:
           do-comparison: 'true'
           # This job's name (so we can find the artifacts)
           job-name: report-size
+          # A token is required to download a previous run's artifact
+          github-token: ${{ secrets.FULL_PERM_TEST }}
   report-download-check:
     runs-on: nixos
     needs: report-size
@@ -41,6 +43,8 @@ jobs:
         uses: https://git.salame.cl/actions/download-artifact@d8d0a99033603453ad2255e58720b460a0555e1e # v4
         with:
           name: report.json
+          github-token: ${{ secrets.ARTIFACT_TOKEN }}
+          run-id: 15
       - name: Verify report exists
         run: |
           cat report.json