gitlab-ci: use runner tag available to those not in fdroid group
This should fix the CI failures that say "No matching runner available". "medium" is the biggest shared runner available to the general public. https://docs.gitlab.com/ee/ci/runners/hosted_runners/linux.html#machine-types-available-for-linux---x86-64
Examples of failures:
- https://gitlab.com/mnbogner/fdroidclient-mnb/-/jobs/8446468813
- https://gitlab.com/Tobias_Groza/fdroidclient/-/jobs/7908875038
@grote @proletarius101 @mnbogner @Tobias_Groza
Also, I added that tag to our own runners.