fix: if statement breaking copy() and ccopy() on macos

[tjex]

Resolves issue #361.

Problem: ccopy and copy return an error on mac, "copy: supported only in the Desktop version"

This is because the cht.sh script checks for presence of the $DISPLAY environment variable, which
is no longer a part of MacOS as it comes from the X11 days. So, calling the function would always
return the error on mac.

pr366 attempted to fix this with a similar approach.

The solution of this PR reuses the $is_macos boolean variable, which is already a part of this
script.

It now only returns the error if $DISPLAY is null/empty and the OS is not MacOS.