chore(deps): update nixos/nix docker tag to v2.26.0

Author: renovate[bot]

Diff for: .gitlab-ci.yml

@@ -66,7 +66,7 @@ before_script:
 
 ci:
   stage: ci
-  image: nixos/nix:2.24.9
+  image: nixos/nix:2.26.0
   script:
     # Cache CI dependencies
     - ./bin/nix-build-and-cache ci
@@ -91,7 +91,7 @@ ci:
 
 artifacts:
   stage: artifacts
-  image: nixos/nix:2.24.9
+  image: nixos/nix:2.26.0
   script:
     - ./bin/nix-build-and-cache just .#static-x86_64-linux-musl
     - cp result/bin/conduit x86_64-linux-musl