... instead of popen to avoid needing shell_exec permission.

Bug: b/179041465
Test: roll_aosp.sh && m && launch Cuttlefish w/ SwANGLE
Test: aosp/1574848 presubmit
Change-Id: Ie64d2730fd2ea00a1b5089459297ef7ffe98bc59
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2676271Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Commit-Queue: Jason Macnak <natsu@google.com>