Use conservative paths to shell bins

bin/mkexec

 
 my $bangs = {
     pl   => `which perl`,
-    sh   => `which sh`,
+    sh   => '/bin/sh',
     py   => `which python`,
-    bash => `which bash`,
+    bash => '/bin/bash',
     sed  => `which sed`,
     bc   => `which bc`
 };