try older Ubuntu

Author: peterbecich

.github/workflows/nix-flake.yml

@@ -12,7 +12,8 @@ jobs:
       fail-fast: true
       matrix:
         os:
-        - ubuntu-latest
+        # - ubuntu-latest
+        - ubuntu-20.04
         # - macos-latest
     name: Nix on ${{ matrix.os }}
     runs-on: ${{ matrix.os }}