fix(ci): don't fail the build if Nix checks fail

as such failures may be due to an out-of-date `Gemfile.nix.lock` and/or
`gemset.nix`.

Author: tomeon

.github/workflows/ci.yml

@@ -78,6 +78,8 @@ jobs:
           bundle exec yard stats --list-undoc
   nix:
     runs-on: ubuntu-latest
+    # Don't fail the build if this job fails
+    continue-on-error: true
     steps:
       - uses: actions/checkout@v3
       - uses: cachix/install-nix-action@v22