Simplify launch via pkexec

- remove usage of env: when timeshift is being launched via pkexec, envs like
  DISPLAY and XAUTHORITY are inherited from the current envoronment
- when they are set explicitly via the binary /usr/bin/env, X server does not permit access,
  the old method of launching did not work on ROSA KDE 4:
  > Could not connect to display
- when /usr/bin/env is launched, the graphical policykit agent dialog shows that /usr/bin/env is being
  launched and does not show TimeShift's icon

There is no reason to use env, I believe. This patch fixes launching on ROSA.

src/timeshift-launcher

@@ -18,7 +18,7 @@ else
 			xhost -SI:localuser:root
 			xhost
 		elif command -v pkexec >/dev/null 2>&1; then
-			pkexec env DISPLAY=$DISPLAY XAUTHORITY=$XAUTHORITY ${app_command}
+			pkexec ${app_command}
 		elif command -v sudo >/dev/null 2>&1; then
 			x-terminal-emulator -e "sudo ${app_command}"
 		elif command -v su >/dev/null 2>&1; then