Prevent $DISPLAY from being overridden in SSH (for X11 forwarding)

When trying to ssh into a qubes vm, `/etc/profile.d/qubes-gui.sh` will
improperly set `$DISPLAY` to be :0. When X11 forwarding is used, `$DISPLAY`
is automatically set to ensure that the relevant windows get forwarded
to the ssh client. However, since qubes-gui.sh is called after a
successful login, the correct value for `$DISPLAY` will be overridden.

This commit fixes the issue by adding a test in qubes-gui.sh to check if
we are currently in an ssh session (by checking if `$SSH_CLIENT` or
`$SSH_TTY` are set).

Note: I do not know .csh scripts and have not tested the proposed change
to qubes-gui.csh, yet.

appvm-scripts/etc/profile.d/qubes-gui.csh

@@ -1,5 +1,7 @@
-setenv DISPLAY ":0"
-setenv _JAVA_AWT_WM_NONREPARENTING "1"
+if (! ($?SSH_CLIENT || $SSH_TTY)) ; then
+  setenv DISPLAY ":0"
+  setenv _JAVA_AWT_WM_NONREPARENTING "1"
+endif
 if ( -f /var/run/qubes-service/software-rendering )
   setenv GSK_RENDERER "cairo"
   setenv GDK_DEBUG "gl-disable vulkan-disable"

appvm-scripts/etc/profile.d/qubes-gui.sh

@@ -1,4 +1,6 @@
-export DISPLAY=:0 _JAVA_AWT_WM_NONREPARENTING=1
+if ! ([ -n "$SSH_CLIENT" ] || [ -n "$SSH_TTY" ] ]); then
+  export DISPLAY=:0 _JAVA_AWT_WM_NONREPARENTING=1
+fi
 if test -f /var/run/qubes-service/software-rendering; then
   export GSK_RENDERER="cairo" GDK_DEBUG="gl-disable vulkan-disable" \
          LIBGL_ALWAYS_SOFTWARE=1