The kernel build requires rsync.

Currently we do not perform CI firmware builds on the next-2102 branch.

Build Gluon for all branches starting with "next" to increase the
coverage of our build tests.

Signed-off-by: David Bauer <mail@david-bauer.net>

* apt is meant for user interactive usage. apt does not guarantee a stable CLI.
* set DEBIAN_FRONTEND=noninteractive to tell apt-get that no user interaction is wanted

apt does not have a stable CLI interface. Don't use it in scripts.

This reverts commit d9621048.

ubuntu-latest is now assigned to Ubuntu 20.04. As we use custom apt
sources for 18.04, pin to this version for now to fix the CI.

The performance benefit the cache brought was due to a broken sources
CDN mirror handling in OpenWrt.

The cache brings no measurable performance benefit. Disable it to slim
down the pipeline steps.

This adds the ability to cache OpenWrt dependencies on a per-target
base. Artifacts over 10MB are excluded, as GitHub imposes a limit of 5G
of available space per repository cache. This affects mostly
linux(-firmware) and gcc / gdb.

The goal is to reduce the total amount of requests necessary to fetch
dependencies.

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.

- Rely on shebang instead of setting shell in workflow
- Run whole install-dependencies.sh in sudo
- Use /bin/sh instead of bash
- set -e

The new step is added with `if: ${{ !cancelled() }}`, so the logs are
stored even when the build fails.

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

This enables build-testing only on master as well as next and release
branches.