Use repository owner instead of actor to log into GitHub CR

[lauft]

On every successful run of branches develop and stable, this GitHub action creates a Docker image with the respective Taskwarrior version. Currently the push to the GitHub container registry (ghcr.io) fails because of access rights.

This PR should fix this by using the repository owner instead of the GitHub actor.

[felixschurk]

Thanks a lot. :)

[lauft]

Currently this image is only (?) used in the task-timewarrior-hook repository, along with the one created for Timewarrior, to test the hook script against the different stable/develop combinations of task and timew.

I wonder whether those images could (should?) be used by users outside the GBF organization, e.g. for testing their TW/TI tools... 🤔

[felixschurk]

@kdheepak
I recall reading something about that the last few days in context to taskwarrior-tui, where the long build time of taskwarrior was taking most of the CI time. I am sorry if I have that somehow wrong.