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/workflows/ci.yml

@@ -58,26 +58,8 @@ jobs:
           nix flake update --commit-lock-file
       - name: Check flake.lock
         uses: DeterminateSystems/flake-checker-action@main
-      # Update the caches daily, flush the cache monthly.
-      - name: Set cache keys
-        id: cache-keys
-        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 }}
+      - name: Cache downloads
+        uses: ./.github/actions/cache-downloads
       - name: nix flake check
         run: nix flake check -L --show-trace
       - name: Build packages for Cachix