Use command over which

What does this Merge Request do?

This MR fixes a few cases that have been reported where the following error occurs when running and gdk <subcommand>:

$ gdk update
❌️ ERROR: Your gdk.yml is invalid.

No such file or directory - which

@alexbuijs and I debugged the problem and found the solution was to replace which with command -v. We did not get to the bottom of why we needed to do this however.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.