trigger.py: Append user tag. This puts the output of 'whoami' into the user tag. This is useful for finding your triggered swarming tasks. Implemented while working on triggering standalone dEQP test runs. Bug: angleproject:5157 Change-Id: Ic850726485a96408c8f2ded36195268eabb275f4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2463370 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>