build(deps): bump cachix/cachix-action from 12 to 13

Bumps [cachix/cachix-action](https://github.com/cachix/cachix-action) from 12 to 13.
- [Release notes](https://github.com/cachix/cachix-action/releases)
- [Commits](cachix/cachix-action@v12...v13)

---
updated-dependencies:
- dependency-name: cachix/cachix-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/docgen.yml

@@ -12,7 +12,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@v23
-      - uses: cachix/cachix-action@v12
+      - uses: cachix/cachix-action@v13
         with:
           name: mrcjkb
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'