Rework CI caching, adding tarball cache

Using the lockfile hash as cache key did not make much sense, since
changes to Unstraightened itself change what we fetch.

And a static restore key means we never flush our cache: that might be
ok if cache size was reasonable, but it is not (over 3 GiB).

Switch to using the date as cache key, and only restore caches from the
current month.

Add tarballs as a second cache.

I considered caching all of ~/.cache/nix, but do not want to find out
the hard way any (new) caches are not portable if I add CI for other
platforms. The tarball-cache seems to be another git tree, so it should
be ok.

.github/workflows/ci.yml

@@ -56,12 +56,26 @@ 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-')"
+          } >> "$GITHUB_OUTPUT"
       - name: Cache git checkouts
         uses: actions/cache@v4
         with:
           path: ~/.cache/nix/gitv3
-          key: nix-gitv3-cache-${{ hashFiles('flake.lock') }}
-          restore-keys: nix-gitv3-cache-
+          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: nix flake check
         run: nix flake check -L --show-trace
       - name: Build packages for Cachix