Switch from shell executor to virtualbox executor.

[a-t-0]

When I run the CI on a device, the tests sometimes break some stuff. (permanently, which could interfere with other tests lateron).

To resolve that, I would like to switch from the shell executor to a virtualbox executor.

https://github.com/TruCol/Self-host-GitLab-CI-for-GitHub/blob/886c5730f27fabcefc28520026f64482aca6faaf/src/install_and_boot_gitlab_runner.sh
line 114:

	sudo gitlab-runner register \
	--non-interactive \
	--url "$gitlab_url" \
	--description $description \
	--registration-token "$runner_token" \
	--executor $executor

[a-t-0]

Note, a docker container instead of virtual machine, or QEMU would also be ok, as long as the tests aren't ran directly on our local GitLab server (Raspberry Pi 4b).