chore(deps): update nixos/nix docker tag to v2.21.1

.gitlab-ci.yml

@@ -54,7 +54,7 @@ before_script:
 
 ci:
   stage: ci
-  image: nixos/nix:2.21.0
+  image: nixos/nix:2.21.1
   script:
     # Cache the inputs required for the devShell
     - ./bin/nix-build-and-cache .#devShells.x86_64-linux.default.inputDerivation
@@ -79,7 +79,7 @@ ci:
 
 artifacts:
   stage: artifacts
-  image: nixos/nix:2.21.0
+  image: nixos/nix:2.21.1
   script:
     - ./bin/nix-build-and-cache .#static-x86_64-unknown-linux-musl
     - cp result/bin/conduit x86_64-unknown-linux-musl