build: fix publish-build-artifacts branch detection

[filipesilva]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
The changes introduced in #13529 pass locally but fail on travis.

More specifically, branch detection in BRANCH=git rev-parse --abbrev-ref HEADresults inBRANCH=HEADinstead ofBRANCH=master` in https://travis-ci.org/angular/angular/jobs/185615697#L2214, which breaks the publishing process further down the script execution.

What is the new behavior?
This PR introduces an alternative mechanism for determining the current branch:

BRANCH=${TRAVIS_BRANCH:-$(git symbolic-ref --short HEAD)}

It will first check for TRAVIS_BRANCH and if not present, determine the branch via a newer git mechanism.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

/cc @chuckjaz

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