Cache big.txt

So we don't have to hit norvig.com 15 times each push :D

.github/workflows/test.yaml

@@ -26,6 +26,11 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           python -m pip install -r requirements.txt
+      - name: Cache test dependencies
+        uses: actions/cache@v3
+        with:
+          key: big-txt
+          path: big.txt
       - name: Test unit tests
         run: |
           python -m unittest discover -s sacremoses/test