build(deps): bump cachix/install-nix-action from 17 to 18 (#23)

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>

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/nix-flake-check.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-      - uses: cachix/install-nix-action@v17
+      - uses: cachix/install-nix-action@v18
         with:
           install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-2.4pre20210604_8e6ee1b/install
           extra_nix_config: |

.github/workflows/nix-flake-update.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
         with:
           install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-2.4pre20210604_8e6ee1b/install
           extra_nix_config: |