build(deps): bump cachix/cachix-action from 14 to 15

Bumps [cachix/cachix-action](https://github.com/cachix/cachix-action) from 14 to 15.
- [Release notes](https://github.com/cachix/cachix-action/releases)
- [Commits](https://github.com/cachix/cachix-action/compare/v14...v15)

---
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/build.yml

@@ -15,7 +15,7 @@ jobs:
           extra-conf: |
             keep-going = true
       - name: Prepare push to binary cache
-        uses: cachix/cachix-action@v14
+        uses: cachix/cachix-action@v15
         if: ${{ github.ref == 'refs/heads/master' }}
         with:
           name: foo-dogsquared