update-deps: Actually prefer GITHUB_EMAIL
update-deps: Actually prefer GITHUB_EMAIL

Follow-up to #1475. This changes the script to *actually* prefer
GITHUB_EMAIL. The change in #1475 cleaned up the email/name logic but it
still checked BUILDKITE_BUILD_CREATOR_EMAIL before GITHUB_EMAIL.

This changes the script to check GITHUB_EMAIL first.

yarpc/yarpc-go,yarpc/yarpc-go