[AIRFLOW-5085] we always pass the default branch name to the build

[potiuk]

TRAVIS_BRANCH is set to TAG when TAG build runs. We should alwayss
use branch and we already have our current branch in
hooks/_default_branch.sh and we can use it from there.

This seems to be the only way as TRAVIS does not pass the branch
in any variable - mainly because we do not know what branch we
are in when building a TAG build

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