Scrub mock chroot before build and force --clean on rebuild

Cancelled jobs can leave the mock chroot in a corrupted state, causing
either 'Failed to resolve user mockbuild' or missing directories like
/var/lib/mock/custom/root/var. Add --scrub=chroot before build and
--clean flag to --rebuild to always start from a fresh chroot.

.forgejo/workflows/build-rpms.yml

@@ -72,9 +72,11 @@ jobs:
           """
           MOCKEOF
           cp /tmp/mock-custom.cfg /etc/mock/custom.cfg
+          mock --root custom --scrub=chroot || true
           mock \
             --root custom \
             --resultdir "${HOME}/mock-results" \
+            --clean \
             --rebuild "${{ env.SRPM_PATH }}"
 
       - name: Upload RPMs to Forgejo Package Registry