Fix mock: use --root with name instead of -r path for mockbuild user

When using -r /path/to/custom.cfg, mock fails to resolve the mockbuild
user because the config isn't in the standard /etc/mock/ directory.
Copy the generated config to /etc/mock/custom.cfg and reference it with
--root custom instead.

.forgejo/workflows/build-rpms.yml

@@ -70,8 +70,9 @@ jobs:
           enabled=1
           """
           MOCKEOF
+          cp /tmp/mock-custom.cfg /etc/mock/custom.cfg
           mock \
-            -r /tmp/mock-custom.cfg \
+            --root custom \
             --resultdir "${HOME}/mock-results" \
             --rebuild "${{ env.SRPM_PATH }}"