Bump cachix/install-nix-action from 14.1 to 15 (#2356)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 14.1 to 15.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@v14.1...v15)

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

Signed-off-by: dependabot[bot] <[email protected]>

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

Author: dependabot[bot]

.github/workflows/nix.yml

@@ -46,7 +46,7 @@ jobs:
     - uses: actions/checkout@v2
       with:
         submodules: true
-    - uses: cachix/install-nix-action@v14.1
+    - uses: cachix/install-nix-action@v15
       with:
         install_url: https://nixos-nix-install-tests.cachix.org/serve/i6laym9jw3wg9mw6ncyrk6gjx4l34vvx/install
         install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'
@@ -81,7 +81,7 @@ jobs:
     - uses: actions/checkout@v2
       with:
         submodules: true
-    - uses: cachix/install-nix-action@v14.1
+    - uses: cachix/install-nix-action@v15
       with:
         install_url: https://nixos-nix-install-tests.cachix.org/serve/i6laym9jw3wg9mw6ncyrk6gjx4l34vvx/install
         install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'