build(deps): bump cachix/install-nix-action from 26 to 28

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 26 to 28.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@v26...V28)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/emacs.yml

@@ -17,7 +17,7 @@ jobs:
  emacs_version:
  - snapshot
  steps:
- - uses: cachix/install-nix-action@v26
+ - uses: cachix/install-nix-action@V28
  with:
  nix_path: nixpkgs=channel:nixpkgs-unstable