CI: move download caching to a composite action

I intend to split the "CI" workflow into a "check" and "build for
cachix" workflow, and both will use this action.

.github/actions/cache-downloads/action.yml

@@ -0,0 +1,40 @@
+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: 'Cache downloads'
+description: "Cache Nix's git and tarball cache in GitHub's action cache."
+runs:
+  using: "composite"
+  steps:
+    # Update the caches daily, flush the cache monthly.
+    - name: Set cache keys
+      id: cache-keys
+      shell: bash
+      run: |
+        {
+          echo "key=$(date +'%Y-%m-%d')"
+          echo "restore=$(date +'%Y-%m-')"
+        } >> "$GITHUB_OUTPUT"
+    - name: Cache git checkouts
+      uses: actions/cache@v4
+      with:
+        path: ~/.cache/nix/gitv3
+        key: nix-gitv3-cache-${{ steps.cache-keys.outputs.key }}
+        restore-keys: nix-gitv3-cache-${{ steps.cache-keys.outputs.restore }}
+    - name: Cache tarballs
+      uses: actions/cache@v4
+      with:
+        path: ~/.cache/nix/tarball-cache
+        key: nix-tarball-cache-${{ steps.cache-keys.outputs.key }}
+        restore-keys: nix-tarball-cache-${{ steps.cache-keys.outputs.restore }}