Replace unknown "tempfile" tool with "mktemp" from coreutils

--- run-mailcap.orig	2017-02-21 20:26:27.367144678 +0100
+++ run-mailcap	2017-02-21 20:25:51.576143081 +0100
@@ -141,9 +141,9 @@
 #   $tmpfile = POSIX::tmpnam($name);
 #   unlink($tmpfile);
 
-    $cmd  = "tempfile --mode=600";
-    $cmd .= " --prefix $head" if $head;
+    $cmd  = "mktemp";
     $cmd .= " --suffix $tail" if $tail;
+    $cmd .= " $head" if $head;
 
     $tmpfile = `$cmd`;
     chomp($tmpfile);
