Use re.search, not re.match, when looking for target_os in MB.
Use re.search, not re.match, when looking for target_os in MB.

Yet another build fix ...

TBR=stevenjb@chromium.org
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1927493002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#389991}

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