Fix previous link in static build pages

Bug #359464 r=justdave

webtools/tinderbox/processbuild.pl

@@ -65,7 +65,8 @@ unlink($lockfile);
 require 'showbuilds.pl';
 # Hardcode static pages to only showing 12 hrs of data
 $nowdate = $maxdate = time;
-$mindate = $maxdate - (12*60*60);
+$hours = 12;
+$mindate = $maxdate - ($hours*60*60);
 print "Changed trees:\n\t@changed_trees\n" if ($debug && $#changed_trees > 0);
 for my $t (@changed_trees) {
     # Override globals used in static page creation