Previously ANGLE would error out when both the enviornment and
the command line args are specified at the same time. Because
the perf bots use both at once we need to handle the conflict
in the same way as the prior test runner does. In this case
the command line takes precedence over the environment.

Bug: angleproject:5124
Change-Id: I1ba765b4e75759922bf9fe2db9f153cfc5995f85
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2489722Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>