build(deps): 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/18cf96c7c98e048e10a83abd92116114cd8504be...ad2ddac53f961de1989924296a1f236fcfbaa4fc)

---
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/basic-eval.yml

@@ -20,7 +20,7 @@ jobs:
     steps:
     - uses: actions/checkout@44c2b7a8a4ea60a981eaca3cf939b5f4305c123b # v4.1.5
     - uses: cachix/install-nix-action@8887e596b4ee1134dae06b98d573bd674693f47c # v26
-    - uses: cachix/cachix-action@18cf96c7c98e048e10a83abd92116114cd8504be # v14
+    - uses: cachix/cachix-action@ad2ddac53f961de1989924296a1f236fcfbaa4fc # v15
       with:
         # This cache is for the nixpkgs repo checks and should not be trusted or used elsewhere.
         name: nixpkgs-ci

.github/workflows/manual-nixos.yml

@@ -22,7 +22,7 @@ jobs:
         with:
           # explicitly enable sandbox
           extra_nix_config: sandbox = true
-      - uses: cachix/cachix-action@18cf96c7c98e048e10a83abd92116114cd8504be # v14
+      - uses: cachix/cachix-action@ad2ddac53f961de1989924296a1f236fcfbaa4fc # v15
         with:
           # This cache is for the nixpkgs repo checks and should not be trusted or used elsewhere.
           name: nixpkgs-ci

.github/workflows/manual-nixpkgs.yml

@@ -24,7 +24,7 @@ jobs:
         with:
           # explicitly enable sandbox
           extra_nix_config: sandbox = true
-      - uses: cachix/cachix-action@18cf96c7c98e048e10a83abd92116114cd8504be # v14
+      - uses: cachix/cachix-action@ad2ddac53f961de1989924296a1f236fcfbaa4fc # v15
         with:
           # This cache is for the nixpkgs repo checks and should not be trusted or used elsewhere.
           name: nixpkgs-ci