fix retest title change

[kwk]

Before the /retest command internally fetched the correct information from the issue in which the /retest <chroot> comment was made. But it didn't use it and accidentally a property (github.issue_title) was used that internally used the current date 🤦 . I've changed the @property to become a regular function and now hopefully we should not see the title being renamed.

NOTE:
You could asked why we edit the title at all and that is a good question. In the past I've experimented with different title formats and I wanted the new format to always show the next a time the check would run.

Fix #538