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/nix.yml

@@ -17,7 +17,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@main
       - uses: DeterminateSystems/magic-nix-cache-action@main
-      - uses: cachix/cachix-action@v14
+      - uses: cachix/cachix-action@v15
         with:
           name: nyx
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"

.github/workflows/update-flake.yml

@@ -21,7 +21,7 @@ jobs:
 
       - uses: DeterminateSystems/nix-installer-action@main
 
-      - uses: cachix/cachix-action@v14
+      - uses: cachix/cachix-action@v15
         with:
           name: nyx
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"

.github/workflows/update-pkgs.yml

@@ -21,7 +21,7 @@ jobs:
 
       - uses: DeterminateSystems/nix-installer-action@main
 
-      - uses: cachix/cachix-action@v14
+      - uses: cachix/cachix-action@v15
         with:
           name: nyx
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"