Fix cache miss in CI

The problem is probably due to a "corrupted" store state.

To fix this, the store will only be cached during the nix-cache job.

Also, crane will be used instead of naersk, due to lack of maintenance and better dependency handling.

.gitlab-ci.yml

@@ -4,11 +4,12 @@
 stages: [ nix-cache, lint, build, tests ]
 
 variables:
+  DEPENDENCIES: ".#theories.inputDerivation .#frontend.inputDerivation .#frontend.cargoArtifacts nixpkgs#gnumake nixpkgs#gnupatch nixpkgs#gnused"
   NIX_STORE_ARTIFACTS_PATH: ".ci_nix_artifacts"
   NIX_STORE_CACHE_PATH: ".ci_nix_cache"
 
 default:
-  image: nixos/nix:2.20.1
+  image: nixos/nix:2.21.2
   interruptible: true
   tags: [ fp ]
 
@@ -18,16 +19,20 @@ default:
    - echo "trusted-public-keys = ${NIX_CI_CACHE_PUB_KEY} cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=" >> /etc/nix/nix.conf
 
    # Since the cache is not guaranteed to be available, it is preferable to build the dependencies for each job (which is no-op if the cache is available)
-   - nix build --print-build-logs .#theories.inputDerivation
-   - nix build --print-build-logs .#frontend.inputDerivation
+   - nix build --print-build-logs ${DEPENDENCIES}
 
-  cache:
+  cache: &nix_cache
     key:
       files: [ flake.lock, flake.nix ]
     paths: [ "${NIX_STORE_CACHE_PATH}" ]
+    policy: pull
 
 
 .build_to_cache:
+  cache:
+    <<: *nix_cache
+    policy: pull-push
+
   script:
    - nix build --print-build-logs ${NIX_DERIVATION}
    - nix store sign --key-file <(echo "${NIX_CI_CACHE_PRIV_KEY}") --recursive ${NIX_DERIVATION}
@@ -54,7 +59,7 @@ default:
 nix-cache:
   stage: nix-cache
   variables:
-    NIX_DERIVATION: .#theories.inputDerivation .#frontend.inputDerivation nixpkgs#gnumake nixpkgs#gnupatch
+    NIX_DERIVATION: ${DEPENDENCIES}
   extends: .build_to_cache
 
 clippy: