Correct JSC build ID handling

[mathiasbynens]

@Constellation reported:

Currently, it is getting buildid from builder (e.g. [1] https://build.webkit.org/api/v2/builders/29/builds?order=-number&limit=1&state_string=build%20successful&complete=true).
But this buildid is not buildsetid, so using buildid with buildsetid API does not work (https://build.webkit.org/api/v2/buildsets/237182).

I think the right way to fix it is retrieving buildrequestid from builder[1], getting buildrequest information via https://docs.buildbot.net/latest/developer/raml/buildrequest.html, and retrieving buildsetid from that.