Do a deep fetch

Currently the build fails with

  warning: reject refs/heads/HEAD because shallow roots are not allowed to be updated
  error: Could not read 0c2088d438
  fatal: Failed to traverse parents of commit ea1803efdc
  error: program 'git' failed with exit code 128

.github/workflows/test.yml

@@ -10,6 +10,8 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
     - uses: cachix/install-nix-action@v8
     #- run: nix flake check
     - run: nix-build -A checks.$(if [[ `uname` = Linux ]]; then echo x86_64-linux; else echo x86_64-darwin; fi)