Rather than relying on the machines we're running tests on having buildbot appropriately installed, we should autoinstall our buildbot dependencies. Twisted makes this somewhat difficult because it's subprocess mechanism breaks our on-demand install system, so we have to acutally install buildbot, Twisted and their dependencies rather than just registering them.
<rdar://problem/104246259>
Pull request: https://github.com/WebKit/WebKit/pull/8646
Committed 262159@main (a5d5c8528495): <https://commits.webkit.org/262159@main> Reviewed commits have been landed. Closing PR #8646 and removing active labels.