Bump cachix/install-nix-action from V28 to 31.3.0 (#1931)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from V28 to 31.3.0. This release includes the previously tagged commit.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Changelog](https://github.com/cachix/install-nix-action/blob/master/RELEASE.md)
- [Commits](cachix/install-nix-action@V28...31.3.0)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-version: 31.3.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/nix.yml

@@ -31,7 +31,7 @@ jobs:
       with:
         submodules: true
     - name: nix
-      uses: cachix/install-nix-action@V28
+      uses: cachix/install-nix-action@31.3.0
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - run: nix develop -c dune build @check @runtest -p merlin-lib,dot-merlin-reader,ocaml-index,merlin