cmd/coordinator: fix trybot remaining builds count for subrepos
cmd/coordinator: fix trybot remaining builds count for subrepos

Fixes assumption broken by https://golang.org/cl/143538 where number
of builders (types of builders) no longer equals the number of trybot
builds. (We now run linux-amd64 1 or 3 times depending on the repo)

Fixes golang/go#28714

Change-Id: I3b85adbb79508890d16311fc75f4b48ffc1f3c78
Reviewed-on: https://go-review.googlesource.com/c/149437
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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