A new compile job can be scheduled from any thread, and
{current_compile_job_} is documented to be protected by {mutex_}. Hence
take the mutex before writing that field.

R=thibaudm@chromium.org, ahaas@chromium.org

Bug: v8:11089
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Change-Id: I2d3b2c51a7d24c7e827bb7ddc9c76b718c2ccb4c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2529140Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71089}