Pin nixpkgs to refs/tags/25.11 and upgrade gitlab-ci to 0.9.0 on lts/ghc-9.14.x

Update nixpkgs input to use pinned tag reference and upgrade gitlab-ci to latest version.

Changes

1. Pin nixpkgs to refs/tags/25.11

   • Change from floating branch reference (nixos-25.11) to pinned tag
   • Ensures reproducible builds across time
   • Prevents unexpected updates from nixpkgs branch changes

2. Upgrade gitlab-ci to 0.9.0

   • Fix for cachix module to push actual outputs instead of .drv files
   • Use om ci run --results=om.json to generate JSON with store paths
   • Extract paths from JSON using jq -r '.result.ROOT.build.outPaths[]'
   • Pipe extracted paths to cachix push via xargs
   • Previous approach failed because omnix only outputs JSON file path to stdout

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