[build-tools] Flag doctor result as warning only for non-0 exit codes

[keith-kurak]

Why

After some updates to the Doctor output, all Doctor steps started showing as warnings on EAS Build logs. It turns out that it was matching on a specific output string in order to declare doctor steps as successful. This may have been necessary in the past, but Doctor now returns an exit code of 1 for any warnings that it would like to alert the user to.
(there are a few other ancillary messages that could be part of the output (such as skipped tests) in an exit code 0 scenario, but none rise to the "alert" level).

How

Removed the string check for "Didn't find any issues with the project". All non-exceptions from the async spawn should not flag as warnings now.

Test Plan

I've never worked in this code and don't know how to setup this repo for testing. However, I think testing would involve:

• Build job with no Doctor warning: no warning on build logs
• Build job with Doctor warnings: warning on build logs