Bump cachix/install-nix-action from 17 to 18

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 17 to 18.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v17...v18)

---
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>

.github/workflows/flake-check.yml

@@ -15,7 +15,7 @@ jobs:
         with:
           # Nix Flakes doesn't work on shallow clones
           fetch-depth: 0
-      - uses: cachix/install-nix-action@v17
+      - uses: cachix/install-nix-action@v18
         with:
           nix_path: nixpkgs=channel:nixos-unstable
       - id: set-matrix
@@ -35,7 +35,7 @@ jobs:
         with:
           # Nix Flakes doesn't work on shallow clones
           fetch-depth: 0
-      - uses: cachix/install-nix-action@v17
+      - uses: cachix/install-nix-action@v18
         with:
           nix_path: nixpkgs=channel:nixos-unstable
       - run: |

.github/workflows/test-develop-flakes.yml

@@ -15,7 +15,7 @@ jobs:
       with:
           # Nix Flakes doesn't work on shallow clones
           fetch-depth: 0
-    - uses: cachix/install-nix-action@v17
+    - uses: cachix/install-nix-action@v18
     - name: Build
       run: nix develop -c bash -c 'meson build && cd build && ninja'
     - name: Run tests

.github/workflows/update-flake-lock.yml

@@ -11,7 +11,7 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v3
       - name: Install Nix
-        uses: cachix/install-nix-action@v17
+        uses: cachix/install-nix-action@v18
         # with:
         #   extra_nix_config: |
         #     access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}