Bump cachix/install-nix-action from 14 to 15

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

---
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/build.yml

@@ -42,7 +42,7 @@ jobs:
     - name: Checkout repository
       uses: actions/checkout@v2.4.0
     - name: Install nix
-      uses: cachix/install-nix-action@v14
+      uses: cachix/install-nix-action@v15
       with:
         nix_path: "${{ matrix.nixPath }}"
         extra_nix_config: |