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/test.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v17
+    - uses: cachix/install-nix-action@v18
       with:
         nix_path: nixpkgs=channel:nixpkgs-unstable
         extra_nix_config: |

.github/workflows/upgrade-flakes.yml

@@ -11,7 +11,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - name: Install Nix
-        uses: cachix/install-nix-action@v17
+        uses: cachix/install-nix-action@v18
         with:
           extra_nix_config: |
             experimental-features = nix-command flakes