For builds set "use_sysroot = true" by default
This is intended to fix a bot config that is hitting a build error with
latest Vulkan validation layer roll.

Bug: angleproject:2560
Change-Id: I06edfc7b74f367bbab23fea6f01b2778b12a470a
Reviewed-on: https://chromium-review.googlesource.com/1068031Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>