Change the default execution strategy for reclient.

Change from 'racing' to 'remote_local_fallback'.  Racing needs more
testing on CQ/CI before making it the default.

Bug: 1149386
Change-Id: I737f27bbd1bcca5b29448882108ad8ec642566ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2635345Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Michael Savigny <msavigny@google.com>
Cr-Commit-Position: refs/heads/master@{#845039}

buildtools/reclient_cfgs/rewrapper_linux.cfg

 platform=container-image=docker://gcr.io/cloud-marketplace/google/rbe-ubuntu16-04@sha256:b4dad0bfc4951d619229ab15343a311f2415a16ef83bcaa55b44f4e2bf1cf635
 server_address=unix:///tmp/reproxy.sock
 labels=type=compile,compiler=clang,lang=cpp
-exec_strategy=racing
+exec_strategy=remote_local_fallback