CI: Use a cache for pre-commit (#10479)

* Use a cache for pre-commit

* Use .pre-commit-config from common

* Move operation until *after* submodule is fetched 🤦

* Use a more descriptive cache key

Author: benjaoming

.circleci/config.yml

@@ -42,13 +42,26 @@ jobs:
       - checkout
       - run: git submodule sync
       - run: git submodule update --init
+      # Recipe: https://pre-commit.com/#managing-ci-caches
+      - run:
+          name: Combine pre-commit config and python versions for caching
+          command: |
+            cp common/pre-commit-config.yaml pre-commit-cache-key.txt
+            python --version --version >> pre-commit-cache-key.txt
+      - restore_cache:
+          keys:
+          - pre-commit-cache-{{ checksum "pre-commit-cache-key.txt" }}
       - run: pip install --user 'tox<5'
       - run: tox -e pre-commit
       - run: tox -e migrations
       - node/install:
           node-version: '10.17.0'
       - run: npm ci
       - run: tox -e eslint
+      - save_cache:
+          key: pre-commit-cache-{{ checksum "pre-commit-cache-key.txt" }}
+          paths:
+          - ~/.cache/pre-commit
 
 workflows:
   version: 2