build(deps): bump cachix/cachix-action from 14 to 15 (#317)

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

---
updated-dependencies:
- dependency-name: cachix/cachix-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>

.github/workflows/nix.yml

@@ -16,7 +16,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@V27
-      - uses: cachix/cachix-action@v14
+      - uses: cachix/cachix-action@v15
         with:
           name: numtide
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
@@ -36,7 +36,7 @@ jobs:
           # Nix Flakes doesn't work on shallow clones
           fetch-depth: 0
       - uses: cachix/install-nix-action@V27
-      - uses: cachix/cachix-action@v14
+      - uses: cachix/cachix-action@v15
         with:
           name: numtide
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
@@ -52,7 +52,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@V27
-      - uses: cachix/cachix-action@v14
+      - uses: cachix/cachix-action@v15
         with:
           name: numtide
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'