Update cache workflow to upload execution reports if failure (#217)

Author: mmcky

.github/workflows/cache.yml

@@ -35,8 +35,15 @@ jobs:
         shell: bash -l {0}
         run: |
           jb build lectures --path-output ./ -W --keep-going
+      - name: Upload Execution Reports
+        uses: actions/upload-artifact@v2
+        if: failure()
+        with:
+          name: execution-reports
+          path: _build/html/reports
       - name: Upload "_build" folder (cache)
         uses: actions/upload-artifact@v2
         with:
           name: build-cache
-          path: _build
+          path: _build
+