build_release_candidate.sh never runs hub command

This is due a True-False inversion bug:
https://github.com/apache/beam/blob/a20bd7e20981f0901b9f4198d6a8c36b12da7be9/release/src/main/scripts/build_release_candidate.sh#L317-L321

(It will always print: "Without hub, you need to create PR manually.")

The PR I got seems legit: https://github.com/apache/beam-site/pull/595/files

Imported from Jira [BEAM-9091](https://issues.apache.org/jira/browse/BEAM-9091). Original Jira may contain additional context.
Reported by: udim.

Inspect release/src/main/scripts/build_release_candidate.sh around lines 317-321 and reproduce the message that says the PR must be created manually. Correct the True-False inversion so the hub command runs when available, then verify the script invokes hub instead of always printing that message.

Written by the indexing model from the issue text.