attempt to fix CI

.github/workflows/nix.yml

@@ -3,10 +3,10 @@ on:
   push:
 jobs:
   tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
-    - uses: cachix/install-nix-action@v8
+    - uses: cachix/install-nix-action@v12
     - uses: numtide/cachix-action@only-cache
       with:
         name: nix-community