Fix gdk update creating an extra "gdk\n" directory.

What does this Merge Request do and why?

With $CDPATH not empty, cd prints the directory it moved to, if the path was in $CDPATH. Since to use $CDPATH to keep it's default behaviour, usually the users add .(current directory) as the top of the $CDPATH, like: .:other/path/after.

Since gdk uses cd $(dirname $0); pwd -P to find the GDK_ROOT path, if the user have value in $CDPATH, the output of the command could be duplicated, generating an extra folder.

Closes: #1400 (closed)

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.