[autoroll] Fix chromium -> angle roller
[autoroll] Fix chromium -> angle roller

Golang's exec package uses PATH via os.Getenv, NOT the PATH variable in
Cmd.Env, when searching for the executable to run.

Bug: angleproject:5114
Change-Id: Ie22c15cc5b8a58c5c56ca862dfec5df32413c3cc
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/403516
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot