Cache Python dependencies in pre-commit workflow

This should speed up installation of pre-commit as part of the Gitea
Action a bit, as it caches the Python packages required for it.

The cache functionality of actions/setup-python is unsuitable for this
use case, as we don't have a file with requirements we want to use as
key. Instead, we just want to cache whatever is downloaded via pip from
PyPI and keep it for further invocations. This commit achieves that by
using the same cache key for every workflow run. The cache is being
updated even if pre-commit fails so we always keep the the Python
packages used for installing pre-commit cached.

.gitea/workflows/pre-commit.yml

@@ -9,4 +9,14 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
+      - id: restore-pip-cache
+        uses: actions/cache/restore@v4
+        with:
+          key: pip-cache-v1-${{ github.workflow }}
+          path: ~/.cache/pip
       - uses: pre-commit/action@v3.0.1
+      - uses: actions/cache/save@v4
+        if: steps.restore-pip-cache.outcome == 'success'
+        with:
+          key: pip-cache-v1-${{ github.workflow }}
+          path: ~/.cache/pip