Prevent `git machete github checkout-prs/anno-prs` from overwriting already present branch annotations

[amalota]

Still need to decide whether it's allowed to add text to the current annotation or if it's not allowed to write at all but i would go with the latter - might be useful in https://github.com/VirtusLab/git-machete/issues/293.

[PawelLipski]

Already addressed on https://github.com/VirtusLab/git-machete/pull/730 right?

[amalota]

Already addressed on #730 right?

No, #730 prevents from overwriting qualifiers but not the whole annotation.