[CI] (pull-requests to) packages repository are not checked

[T-X]

It was noticed that for freifunk-gluon/community-packages#151 more CI complaints were happening than for the equivalent PRs for the main repository here, even with the .shellcheckrc adopted:

• Adding ffhl-gluon-mesh-batman-adv-brmldproxy community-packages#151
• brmldproxy: add package packages#264

It seems like we should add some CI checks to the packages repository to be on par with the community-packages repository?

[rotanid]

@T-X thanks to @neocturne the linting is now updated, please check/update your pull requests accordingly :)