x/build: coordinator shows trybot successful builds as still active

[bradfitz]

There's some bug with the interaction of trybots and them succeeding in the build coordinator.

Look at the screenshot: the builds are done, but they're in the "active" section.

Further, down the page it says:

*** trybot state
Change-ID: I5f58aa58e74bbe4ec91b2e9b8c81921338053b00 Commit: 160f5ff2a7fad1fafa6cd817d64ebf488bd2d1fd
   Remain: 0, fails: []
  linux-386: running=false
  linux-amd64: running=false
  linux-amd64-race: running=false
  freebsd-386-gce101: running=false
  freebsd-amd64-gce101: running=false
  windows-386-gce: running=false
  windows-amd64-gce: running=false
  openbsd-386-gce56: running=false
  openbsd-amd64-gce56: running=false
  plan9-386-gcepartial: running=false

/cc @adg