ci: change cache action to v2

Supposed to be faster.

.github/workflows/main.yml

@@ -166,7 +166,7 @@ jobs:
     - uses: actions/setup-python@v2
     - name: set PY
       run: echo "name=PY::$(python -c 'import hashlib, sys;print(hashlib.sha256(sys.version.encode()+sys.executable.encode()).hexdigest())')" >> $GITHUB_ENV
-    - uses: actions/cache@v1
+    - uses: actions/cache@v2
       with:
         path: ~/.cache/pre-commit
         key: pre-commit|${{ env.PY }}|${{ hashFiles('.pre-commit-config.yaml') }}