Bump cachix/install-nix-action from 30 to 31

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 30 to 31.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@v30...v31)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/update-ghc.yaml

@@ -22,7 +22,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           ref: master
-      - uses: cachix/install-nix-action@v30
+      - uses: cachix/install-nix-action@v31
         with:
           nix_path: nixpkgs=nixpkgs/default.nix
       - name: Fetch updates

.github/workflows/workflow.yaml

@@ -71,7 +71,7 @@ jobs:
         with:
           path: ~/repo-cache
           key: repo-cache-${{ runner.os }}-nixpkgs-${{ env.cache-version }}
-      - uses: cachix/install-nix-action@v30
+      - uses: cachix/install-nix-action@v31
         with:
           nix_path: nixpkgs=./nixpkgs/default.nix
           extra_nix_config: |