Bump cachix/install-nix-action from 31.8.4 to 31.9.0 (#416)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 31.8.4 to 31.9.0.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Changelog](https://github.com/cachix/install-nix-action/blob/master/RELEASE.md)
- [Commits](cachix/install-nix-action@0b0e072...4e002c8)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-version: 31.9.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

.github/workflows/ci-nix.yml

@@ -26,7 +26,7 @@ jobs:
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@v6
-      - uses: cachix/install-nix-action@0b0e072294b088b73964f1d72dfdac0951439dbd # v31.8.4
+      - uses: cachix/install-nix-action@4e002c8ec80594ecd40e759629461e26c8abed15 # v31.9.0
         with:
           extra_nix_config: |
             sandbox = true