migrate mobiletc-prebuild to crostc-worker

As mentioned in b/445451590, the mobiletc-prebuild account should no
longer be in use; the new account its CLs are uploaded under is
crostc-worker.

Migrate here, since the rationale for MergeApprovedYolo exemptions still
applies.

Clone of http://crrev.com/i/8602883

Bug: 445451590
Change-Id: I737ada14056b5446c08be4da999c11f332edb83f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/+/6969092
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>