cuirass: Fix 'show-help' argument.

* bin/cuirass.in (show-help): Remove reference to unbound variable
'program-name'.  Turn PROG into a mandatory argument.

bin/cuirass.in

@@ -27,7 +27,7 @@ exec ${GUILE:-@GUILE@} --no-auto-compile -e main -s "$0" "$@"
              (ice-9 getopt-long)
              (ice-9 match))
 
-(define* (show-help #:optional (prog (program-name)))
+(define* (show-help prog)
   (simple-format #t "Usage: ~a [OPTIONS] [CACHEDIR]" prog)
   (display "
 Run Guix job from a git repository cloned in CACHEDIR.