frontend: fix delete multiple builds
Exposed by beaker tests, backend complaints:
Traceback (most recent call last):
File "/usr/bin/copr-backend-process-action", line 35, in handle_action
result = action.run()
File "/usr/lib/python3.9/site-packages/copr_backend/actions.py", line 345, in run
appstream = ext_data["appstream"]
KeyError: 'appstream'
Fixes 3c765a95f21448e28a8d41b6655f89a1c126c426