buildmaster: fix unpinned buildbot package version

buildmaster: fix unpinned buildbot package version

It seems, that current pip3 install pinning doesnt work properly as it
can result in the following package versions installation:

 buildbot-3.6.1 buildbot-console-view-3.5.0 buildbot-grid-view-3.5.0
 buildbot-waterfall-view-3.5.0 buildbot-worker-3.5.0 buildbot-www-3.5.0

So lets pin the buildbot package itself to BUILDBOT_VERSION as well.

References: https://gitlab.com/openwrt/buildbot/-/jobs/3324049571#L1158
Signed-off-by: Petr Štetiar <ynezz@true.cz>