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/pr.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
       with:
         nix_path: nixpkgs=channel:nixos-unstable
         # nix 2.6 breaks restrict-eval, when using the NIX_PATH

.github/workflows/update.yml

@@ -14,7 +14,7 @@ jobs:
     - uses: actions/checkout@v3
       with:
         fetch-depth: '0'
-    - uses: cachix/install-nix-action@v17
+    - uses: cachix/install-nix-action@v18
       with:
         nix_path: nixpkgs=channel:nixos-unstable
         # nix 2.6 breaks restrict-eval, when using the NIX_PATH
@@ -31,7 +31,7 @@ jobs:
     needs: update_nur
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v17
+    - uses: cachix/install-nix-action@v18
       with:
         nix_path: nixpkgs=channel:nixos-unstable
         extra_nix_config: |