Add ubuntu back to matrix, remove nix flake check

Need to consider which systems to support instead of using
`flake-utils.lib.eachDefaultSystem` since the DFINITY SDK and
`nixpkgs-dfinity-sdk` doesn't support everything.

paulyoung/nixpkgs-dfinity-sdk#1

.github/workflows/nix.yml

@@ -7,10 +7,9 @@ jobs:
     name: Nix on ${{ matrix.os }}
     strategy:
       matrix:
-        os: [macos-latest]
+        os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/[email protected]
     - uses: cachix/install-nix-action@v15
     - run: nix build --show-trace
-    - run: nix flake check