bitrisescript: support re-connecting to a Bitrise build

[ahal]

In certain circumstances (e.g unexpected worker shutdown), it would be nice if a task could "re-connect" to a running Bitrise build. Bitrise has an API to list builds, so it should be sufficient to query this and check if a build has already been submitted for the current revision / workflow / build params.

This way we don't trigger a new build in Bitrise every time the Taskcluster task retries.

See also #971 . There are cases where we'll want to abort the build in Bitrise and there are cases we'll want to leave it running so we can re-connect later.