Part of [ENG-2457](https://linear.app/comm/issue/ENG-2457).
We want to be able to build the desktop app on Windows but currently, the `postinstall` script of `native` workspace doesn't work on Windows:
- it uses the bash syntax (and on windows `postinstall` runs under `cmd.exe`)
- there are some problems with paths when it comes to `patch-package` because of some symlink weirdness in node_modules
This diff moves the postinstall logic into a separate script file and runs it with `bash` explicitly. If we are on windows we skip everything (we are only going to be building the desktop app).
This will work on windows provided there's a bash executable in PATH, thankfully that's true for windows github runners (bash is included in the git for windows).
Additionally, we skip `pod install` if we aren't on macOS instead of using `|| true` which hides errors.