upload all devshell inputs to the cache

This will also include attic, so we don't need to explicitly do this
in `./bin/nix-build-and-cache` anymore, which is good because that
script gets called a good number of times and doing that repeatedly was
a bit of a waste.

.gitlab-ci.yml

@@ -36,6 +36,9 @@ ci:
   stage: ci
   image: nixos/nix:2.20.4
   script:
+    # Cache the inputs required for the devShell
+    - ./bin/nix-build-and-cache .#devShells.x86_64-linux.default.inputDerivation
+
     - direnv exec . engage
   cache:
     key: nix