Fix identifying main branch

For example here https://buildbot.mariadb.org/#/builders/370/builds/132, the main branch is wrongly identified as 10.1 instead of 10.10.

The identification of the regex needs to be adjusted:

https://github.com/MariaDB/buildbot/blob/cd5378a7b6549e3bf5930306c2eed29239aa3a38/master-libvirt/master.cfg#L180
and
https://github.com/MariaDB/buildbot/blob/cd5378a7b6549e3bf5930306c2eed29239aa3a38/master-libvirt/master.cfg#L168