Fix luci.builder throws "cannot insert into frozen hash table" error in
some cases.

R=tandrii, vadimsh

Bug: 1252321
Change-Id: I6dac50ea77efcaa8d78fe274bba54d428108a003
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3195270
Auto-Submit: Weiwei Lin <weiweilin@google.com>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Weiwei Lin <weiweilin@google.com>