run-script: Fix conflicts with DISPLAY system wide variable

`DISPLAY` is a well-known system-wide
variable used in Linux for GUI applications.
Overwriting it in the script could lead to
unintended side effects, especially when launching
graphical applications. Renaming it to HDLCD_GUI to
avoid such conflicts.

Signed-off-by: Ahmed Ismail <Ahmed.Ismail@.arm.com>