Simplify dev.nix (#2267)

I was being over ambitious with attempting to automatically `flutter pub
get` all the things. Turns out, can't subshell in `dev.nix`

## Pre-launch Checklist

- [ ] I read the [Effective Dart: Style] _recently_, and have followed
its advice.
- [ ] I signed the [CLA].
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Effective Dart: Style]:
https://dart.dev/guides/language/effective-dart/style
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md

.idx/dev.nix

@@ -21,7 +21,7 @@
     workspace = {
       # Runs when a workspace is first created
       onCreate = {
-        updateStable = "flutter channel stable && flutter upgrade && for dir in `find . -name pubspec.yaml -exec dirname {} \; | sort`; do pushd $dir ; flutter pub get; popd; done";
+        updateStable = "flutter channel stable && flutter upgrade";
       };
     };
   };