add action

action.yml

@@ -0,0 +1,12 @@
+name: "Nix preparation"
+description: "Prepares the nix shell for the running CI"
+
+runs:
+  using: composite
+  steps:
+    - uses: actions/checkout@v4
+      with:
+        ref: ${{ github.head_ref }}
+    - uses: cachix/install-nix-action@v22
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable