workflows/{check-shell,lib-tests}: use nixpks CI cachix cache

Any change to shell.nix or ci/ currently needs to rebuild treefmt on 4
systems from scratch. We can avoid that by using the existing cachix
cache.

Adding the cache to lib-tests won't do much on it's own, yet, but it
will with the next commit.

.github/workflows/check-shell.yml

@@ -44,5 +44,11 @@ jobs:
 
       - uses: cachix/install-nix-action@526118121621777ccd86f79b04685a9319637641 # v31
 
+      - uses: cachix/cachix-action@0fc020193b5a1fa3ac4575aa3a7d3aa6a35435ad # v16
+        with:
+          # This cache is for the nixpkgs repo checks and should not be trusted or used elsewhere.
+          name: nixpkgs-ci
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+
       - name: Build shell
         run: nix-build untrusted/ci -A shell

.github/workflows/lib-tests.yml

@@ -32,6 +32,12 @@ jobs:
         with:
           extra_nix_config: sandbox = true
 
+      - uses: cachix/cachix-action@0fc020193b5a1fa3ac4575aa3a7d3aa6a35435ad # v16
+        with:
+          # This cache is for the nixpkgs repo checks and should not be trusted or used elsewhere.
+          name: nixpkgs-ci
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+
       - name: Building Nixpkgs lib-tests
         run: |
           nix-build untrusted/ci -A lib-tests