cmd/coordinator/buildongce: don't curl -o directly to the coordinator binary
cmd/coordinator/buildongce: don't curl -o directly to the coordinator binary

It might still be running as the watcher. Use install(1) instead.

Updates golang/go#12631

Change-Id: I5c2b020b7c19ee0e912e345a03d21353b237fe61
Reviewed-on: https://go-review.googlesource.com/14586
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

proppy/build,golang/build,golang/build,proppy/build,proppy/build,golang/build,golang/build,proppy/build