coordinator: when retrying an aborted try build, remember the trySet

This line got dropped in recent the buildlet pool abstraction change.

Fixes golang/go#10308

Change-Id: I4a6455cafd823b8d6abb9fc7ac68296925c70feb
Reviewed-on: https://go-review.googlesource.com/8313
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/coordinator/coordinator.go b/cmd/coordinator/coordinator.go
index 356554b..8314b01 100644
--- a/cmd/coordinator/coordinator.go
+++ b/cmd/coordinator/coordinator.go
@@ -711,6 +711,7 @@
 		}
 		brev := builderRev{name: bconf.Name, rev: ts.Commit}
 		bs, _ = newBuild(brev)
+		bs.trySet = ts
 		go bs.start()
 		ts.mu.Lock()
 		ts.builds[idx] = bs