cmd/coordinator: cleanup reverse buildlet net.Conn
cmd/coordinator: cleanup reverse buildlet net.Conn

Avoids an FD leak. Tested by hand with lsof.

Fixes golang/go#10698 (probably)

Change-Id: Ic81db7eb504da9f1d9e4fa0518792c1e02da21cc
Reviewed-on: https://go-review.googlesource.com/9742
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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