Hide xcodebuild stderr to solve #21672

[lacostej]

See #21672

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I see several green ci/circleci builds in the "All checks have passed" section of my PR (connect CircleCI to GitHub if not)
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.
• I've added or updated relevant unit tests.

Motivation and Context

Description

I haven't found a way to get rid of those issues, so hiding them by merging them with stdout. It doesn't affect the parsing of the settings as no stderr line contains a =.

Testing Steps

[AliSoftware]

This hides all of stderr logs though, without an option to see them. And iinm, while we tee the stdout to a file especially to allow people to still browse the full log even when they pipe thru xcpretty / xcbeautify, we don't have an equivalent for being able to browse stderr messages optionally if they're masked in the streamed output.

---

[EDIT] I just realized that that patch is only applied to the command that generates -showBuildSettings, so I think that's ok; as I don't expect much valuable info from stderr on that specific command. Just wanted to make sure we don't swallow stderr on the regular xcodebuild build commands like we'd do for showBuildSettings, but that seems to be good here 👍