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](cachix/install-nix-action@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>

Author: dependabot[bot]

.github/workflows/ci.yaml

@@ -40,7 +40,7 @@ jobs:
         push: true
         tags: localhost:5000/tinkerbell/hook-docker:0.0
 
-    - uses: cachix/install-nix-action@v14
+    - uses: cachix/install-nix-action@v15
       with:
         nix_path: nixpkgs=channel:nixos-unstable
 

.github/workflows/push.yaml

@@ -40,7 +40,7 @@ jobs:
         push: true
         tags: quay.io/tinkerbell/hook-docker:0.0
 
-    - uses: cachix/install-nix-action@v14
+    - uses: cachix/install-nix-action@v15
       with:
         nix_path: nixpkgs=channel:nixos-unstable