For https://github.com/flutter/flutter/issues/147750.
Depends on https://github.com/flutter/buildroot/pull/852.
This PR rolls forward reclient, the buildroot, and our RBE configs. The
new RBE configs do a few things. First they default the exec strategy to
`racing`. Second, they add a local wrapper script that passes the
correct value for `-fdebug-prefix-map` in both the local and remote
cases. Finally, they remove the canonicalization of the build's working
directory, so that the remote working directory will have the same name
as the local working directory. This solves the issue where the build
working directory contained a mysterious `set_by_reclient/a` component.
f1ac521788