AppImage: AppRun use #!/usr/bin/env bash

There are Linux systems without bash installed at /bin/sh or it is
in a non-standard location.

appimage/rsc/AppRun

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 export CURDIR=$(dirname $(readlink -f "${0}"))
 
 export PATH="${CURDIR}/bin":$PATH