buildlet: fix the likely source of our coordinator fd leaks
buildlet: fix the likely source of our coordinator fd leaks

Fixes golang/go#10062

Change-Id: Ifa84c9acca347e84c38a4bbae2ad16769bf3e318
Reviewed-on: https://go-review.googlesource.com/6702
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

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