ci: update nix action (#888)

* ci: update nix action

* fix

Author: anmonteiro

.github/workflows/nix.yml

@@ -12,12 +12,9 @@ concurrency:
 jobs:
   tests:
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        ocamlVersion: [5_2]
     steps:
     - uses: actions/checkout@v4
-    - uses: cachix/install-nix-action@v27
+    - uses: cachix/install-nix-action@v31
       with:
         extra_nix_config: |
           extra-substituters = https://anmonteiro.nix-cache.workers.dev