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.

[abitrolly]

Does MacOS expose $XDG_CURRENT_DESKTOP variable?
Is it possible at all to run cht.sh script on MacOS without desktop?
If yes, then is clipboard copy possible in this case?

https://superuser.com/questions/1074068/what-is-the-difference-between-desktop-session-xdg-session-desktop-and-xdg-cur

[tjex]

I'm not following the reasoning for your questions. Should they apply to the pr or are they general questions?

[abitrolly]

We need to detect if clipboard is available. It probably only available if script is run in the desktop session. The way to detect desktop session on Linux is using $XDG_CURRENT_DESKTOP variable. The questions are about how it works on MacOS.

[tjex]

ah ok. Well as far as I found out, $DESKTOP used to exist as an env variable on mac. But not any more. I would say that as osx effectively is a desktop environment (there is no headless / minimal install or such) then it's redundant to check? Another option could just be to check $(which pbcopy)?