Do not mix two different HTTP libraries in one script. This fixes a
warning:

/var/lib/jenkins/slave.py:82: DeprecationWarning: URLopener style of invoking requests is deprecated. Use newer urlopen functions/methods

This simple implementation has the disadvantage that it stores the whole
file in RAM, but this isn't an issue for the 1.5MB slave.jar.

Fixes a warning:

/usr/local/lib/python3.7/dist-packages/urllib3/connectionpool.py:1004: InsecureRequestWarning: Unverified HTTPS request is being made to host 'build.ffh.zone'. Adding certificate verification is strongly advised. See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings

This allows us to make use of builders that can't run virtual machines.

Drop the TMUX environemnt variable, it's not needed anymore.

This will hopefully make CI runs more robust and recover stale workers.

This is required to build the next branch which requires rsync to be
available.

Rsync is a requirement for OpenWrt master and therefore our next
branch. Currently builds on x86-64 error out due to missing rsync:

/bin/sh: 1: rsync: not found