Bump cachix/cachix-action from 14 to 15 (#1084)

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] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/nix-action-coq-8.16-macos.yml

@@ -23,7 +23,7 @@ jobs:
       with:
         nix_path: nixpkgs=channel:nixpkgs-unstable
     - name: Cachix setup metacoq
-      uses: cachix/cachix-action@v14
+      uses: cachix/cachix-action@v15
       with:
         authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
         extraPullNames: coq, coq-community
@@ -63,7 +63,7 @@ jobs:
       with:
         nix_path: nixpkgs=channel:nixpkgs-unstable
     - name: Cachix setup metacoq
-      uses: cachix/cachix-action@v14
+      uses: cachix/cachix-action@v15
       with:
         authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
         extraPullNames: coq, coq-community
@@ -108,7 +108,7 @@ jobs:
       with:
         nix_path: nixpkgs=channel:nixpkgs-unstable
     - name: Cachix setup metacoq
-      uses: cachix/cachix-action@v14
+      uses: cachix/cachix-action@v15
       with:
         authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
         extraPullNames: coq, coq-community

.github/workflows/nix-action-coq-8.16-ubuntu.yml

@@ -23,7 +23,7 @@ jobs:
       with:
         nix_path: nixpkgs=channel:nixpkgs-unstable
     - name: Cachix setup metacoq
-      uses: cachix/cachix-action@v14
+      uses: cachix/cachix-action@v15
       with:
         authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
         extraPullNames: coq, coq-community
@@ -63,7 +63,7 @@ jobs:
       with:
         nix_path: nixpkgs=channel:nixpkgs-unstable
     - name: Cachix setup metacoq
-      uses: cachix/cachix-action@v14
+      uses: cachix/cachix-action@v15
       with:
         authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
         extraPullNames: coq, coq-community
@@ -108,7 +108,7 @@ jobs:
       with:
         nix_path: nixpkgs=channel:nixpkgs-unstable
     - name: Cachix setup metacoq
-      uses: cachix/cachix-action@v14
+      uses: cachix/cachix-action@v15
       with:
         authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
         extraPullNames: coq, coq-community