Fix mock: override root name after include to prevent cache collision

The included alma+epel-10-x86_64.cfg sets config_opts['root'], which
overrides --root custom and causes mock to reuse the old cached chroot
where mockbuild user was missing. Explicitly set config_opts['root'] after
the include to force a fresh custom root.

.forgejo/workflows/build-rpms.yml

@@ -52,6 +52,7 @@ jobs:
         run: |
           cat > /tmp/mock-custom.cfg <<'MOCKEOF'
           include('/etc/mock/alma+epel-10-x86_64.cfg')
+          config_opts['root'] = 'custom'
           config_opts['yum.conf'] += """
           [sonicde-rpm]
           name=SonicDE RPM