Run gbp setup-gitattributes after gbp pull

A couple of projects whose extract-source job fails because of gbp setup-gitattributes:

• https://salsa.debian.org/nvidia-team/nvitop/-/jobs/4857558
• https://salsa.debian.org/carnil/boxes/-/jobs/4866410

@siretart: Is it sensible to run gbp setup-gitattributes after gbp pull?

/cc @carnil: This change hopefully fixes the issue with boxes.

Closes #322 (closed)