build(deps): bump cachix/cachix-action in /.github/actions/install-nix

Bumps [cachix/cachix-action](https://github.com/cachix/cachix-action) from 12 to 15.
- [Release notes](https://github.com/cachix/cachix-action/releases)
- [Commits](cachix/cachix-action@v12...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/actions/install-nix/action.yml

@@ -12,7 +12,7 @@ runs:
       extra-conf: |
         accept-flake-config = true
   - uses: DeterminateSystems/magic-nix-cache-action@v8
-  - uses: cachix/cachix-action@v12
+  - uses: cachix/cachix-action@v15
     continue-on-error: true
     with:
       name: bytecodealliance