ci: make dependency guards portable to macOS Bash (#3751)

## Summary

Make the dependency workflow's guards behave consistently on macOS and
Linux so contributors can run the same checks locally that run in CI.

macOS ships Bash 3.2, which does not stop at a failed `[[ ... ]]` guard
just because `set -e` is enabled. This causes three existing rejection
tests to fail locally even though they pass on Linux. Exit explicitly
when a guard fails, keeping the accepted base SHAs and repository URLs
unchanged.