continue-on-error in "Download debs" step

As we auto-enable DOWNLOAD_DEBS on subsequent builds, we should gracefully
handle the failure case that the first build didn't upload any debs.
Due to external changes, the next build might succeed anyway.

.github/workflows/build.yaml

@@ -122,6 +122,7 @@ jobs:
       - name: Download debs from previous run
         uses: actions/download-artifact@v3
         if: inputs.DOWNLOAD_DEBS
+        continue-on-error: true
         with:
           name: debs
           path: ${{ env.DEBS_PATH }}