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/cachix.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-      - uses: cachix/install-nix-action@v17
+      - uses: cachix/install-nix-action@v18
       - name: Cachix
         uses: cachix/cachix-action@v10
         with:

.github/workflows/update.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-      - uses: cachix/install-nix-action@v17
+      - uses: cachix/install-nix-action@v18
       - name: Cachix
         uses: cachix/cachix-action@v10
         with: