Cache the output of HTMLProofer to avoid hitting rate limits

Author: julienrf

.github/workflows/ci.yml

@@ -35,4 +35,11 @@ jobs:
              --allow-hash-href=true\
              --enforce-https=false\
              --ignore-missing-alt=true\
-             --swap-urls "https\:\/\/scala\.epfl\.ch:"
+             --swap-urls "https\:\/\/scala\.epfl\.ch:"\
+             --cache '{ "timeframe": { "external": "30d" } }'
+      - name: Cache HTMLProofer
+        id: cache-htmlproofer
+        uses: actions/cache@v2
+        with:
+          path: tmp/.htmlproofer
+          key: ${{ runner.os }}-htmlproofer