Improve script compatibility

Now the script is conforms to POSIX shell script which is universal on
all linux.
Also make it executable.

.github/workflows/helper/appimage/export_vars.sh

@@ -1,10 +1,12 @@
+#!/bin/sh
+
 # this file is called from AppRun so 'root_dir' will point to where AppRun is
 root_dir="$(readlink -f "$(dirname "$0")")"
 
 # Insert the default values because after the test we prepend our path
 # and it will create problems with DEs (eg KDE) that don't set the variable
 # and rely on the default paths
-if [[ -z ${XDG_DATA_DIRS} ]]; then
+if [ -z "${XDG_DATA_DIRS}" ]; then
     XDG_DATA_DIRS="/usr/local/share/:/usr/share/"
 fi