Draft: Add user's existing PATH at the very end

What does this Merge Request do and why?

This MR appends the user's ${PATH} variable at the end of the built PATH= variable to ensure any binaries outside of the 'standard' paths are included. I'd prefer to not do this because we lose a lot more control over the execution environment which opens the door for random issues such as MacPorts etc but it has to be done.

!2321 (merged) brought in the concept of the .gdkrc file.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.