Intermittent failure to install native OSX dependencies in official builds

[sbomer]

See the failing OSX build in https://dnceng.visualstudio.com/internal/_build/results?buildId=68976.
The error is as follows, during a brew install:

2019-01-09T13:02:12.5324730Z /usr/local/Homebrew/Library/Homebrew/config.rb:39:in `initialize': no implicit conversion of nil into String (TypeError)
2019-01-09T13:02:12.5325920Z 	from /usr/local/Homebrew/Library/Homebrew/config.rb:39:in `new'
2019-01-09T13:02:12.5326790Z 	from /usr/local/Homebrew/Library/Homebrew/config.rb:39:in `<top (required)>'
2019-01-09T13:02:12.5327660Z 	from /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
2019-01-09T13:02:12.5328500Z 	from /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
2019-01-09T13:02:12.5329490Z 	from /usr/local/Homebrew/Library/Homebrew/global.rb:25:in `<top (required)>'
2019-01-09T13:02:12.5330250Z 	from /usr/local/Homebrew/Library/Homebrew/brew.rb:13:in `require_relative'
2019-01-09T13:02:12.5330940Z 	from /usr/local/Homebrew/Library/Homebrew/brew.rb:13:in `<main>'

[chcosta]

@dotnet/dnceng is this a known issue?

[sbomer]

I have some experimental evidence that running brew update separately before brew install (as opposed to letting brew install do the auto-update) works around the issue.

[sbomer]

Apparently other teams are hitting this as well, and it was caused by Homebrew/brew@9156d6d. See Homebrew/brew#5513 for more info. Workarounds include setting HOMEBREW_LOGS and running brew update. It sounds like the mac images will eventually be updated to a homebrew with the fix.

[RussKeldorph]

Tracks reverting dotnet/coreclr#21913.

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