Fix running dotnet UI tests out of vscode/in CI

[alehander92]

talked with Franz about it

1. fix: use a valid nixos binary for node for ui tests in playwright

we were getting something like
```
Could not start dynamically linked executable: /home/alexander92/codetracer/ui-tes
NixOS cannot run dynamically linked executables intended for generic
linux environments out of the box. For more information, see:
https://nix.dev/permalink/stub-ld
```
when trying to run `dotnet run` outside of vscode (at least for me, alexander,
it seems it was running ok in vscode or at least through an extension for Franz):
electron was opening, but test code wasn't
really running

now one needs to run `./dotnet_build.sh` now instead of `dotnet build`
(and `dotnet run` directly without ever running the previous command might not
work)

2. ci: try to run ui dotnet playwright tests in CI 
    (EDIT: trying to run it after `just build-nix`, but still a problem: left unresolved)
   

TODO: maybe use dotnet test instead of dotnet run, and a test runner, if it makes sense? probably not in scope for this PR (EDIT: left for a future PR)

[alehander92]

merging: hopefull useful