ci: add nix install action (#324)

Author: NickLarsenNZ

.github/workflows/generate_prs.yml

@@ -13,6 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # tag=v4.1.1
+      - uses: cachix/install-nix-action@6004951b182f8860210c8d6f0d808ec5b1a33d28 # tag=v25
       - name: Install Ansible
         env:
           DEBIAN_FRONTEND: noninteractive