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/+/2463370Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>