Avoid ERR trap activating if there's no `brew` command

When generating SHA2 checksums with `openssl`, we want to give
preference to the newer OpenSSL that might have been installed with
Homebrew. However if there's no `brew` command the ERR trap will kick in
and display the "BUILD FAILED" notice although the build would proceed
to run.

Related: c0dc8908

Fixes #713