packaging: Avoid using the file command

There is no need to execute `file` in order to determine if a given file
is an executable, the bulitin -x test is enough.

Change-Id: I190571c5b36f08772fc795eca9fbd082ba248c04
Signed-off-by: Ofer Schreiber <oschreib@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1029036