fix: pr workflow

.github/workflows/pr.yaml

@@ -9,12 +9,12 @@ concurrency:
   group: ${{ github.actor }}
 jobs:
   test:
+    container:
+      image: nixos/nix:latest
     runs-on: self-hosted
     steps:
     - name: Clone
       uses: actions/checkout@v2
-    - name: Install Nix
-      uses: cachix/install-nix-action@v15
     - name: Build
       run: |
         nix build --print-build-logs -v