use nothing-but-nix to give GitHub Action more disk

https://github.com/wimpysworld/nothing-but-nix

haskell#1391 (comment)

Author: peterbecich

.github/workflows/nix-flake.yml

@@ -19,6 +19,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4
+    - uses: wimpysworld/nothing-but-nix@main
     - uses: cachix/install-nix-action@v31
       with:
         extra_nix_config: |