update: prefix rm call with command in trap (#9107)

* Suppress the problematic trap output in check_upg

The newly added trap, in systems where `rm` is aliased to `rm="rm -v"`,
shows a message stating that "update.lock" has been removed each time `zsh` is called.
I simply suppressed it with directing the output to `/dev/null`.

* Use `command` instead of >/dev/null to suppress