Fix path to node in .xcode.env.local (#43333)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/43333

This change fixes https://github.com/facebook/react-native/issues/43285.
Basically, when using a `yarn` alias to install pods, yarn creates a copy of the `node` and `yarn` executables and the `command -v node` command will return the path to that executable.

## Changelog
[iOS][Fixed] - Do not use temporary node when creating the .xcode.env.local

Reviewed By: dmytrorykun

Differential Revision: D54542774

fbshipit-source-id: 3ab0d0bb441988026feff9d5390dcfd10869a1b5