Bump cachix/cachix-action from 14 to 15 in the actions-deps group (#1727)

Author: dependabot[bot]

.github/workflows/check-ios-android-bindings.yml

@@ -30,7 +30,7 @@ jobs:
           # Mostly to avoid GitHub rate limiting
           extra_nix_config: |
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
-      - uses: cachix/cachix-action@v14
+      - uses: cachix/cachix-action@v15
         with:
           name: xmtp
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
@@ -63,7 +63,7 @@ jobs:
           # Mostly to avoid GitHub rate limiting
           extra_nix_config: |
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
-      - uses: cachix/cachix-action@v14
+      - uses: cachix/cachix-action@v15
         with:
           name: xmtp
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'