Fix non-zero exit code even if the script succeed (#1302)

The script succeeded but the exit code is 1 because it's not a dryrun so the last command failed.
diff --git a/scripts/build_ngx_pagespeed.sh b/scripts/build_ngx_pagespeed.sh
index 7ae8bfa..edf3314 100755
--- a/scripts/build_ngx_pagespeed.sh
+++ b/scripts/build_ngx_pagespeed.sh
@@ -524,7 +524,9 @@
     echo "You'll also need to configure ngx_pagespeed if you haven't yet:"
     echo "  https://developers.google.com/speed/pagespeed/module/configuration"
   fi
-  "$DRYRUN" && echo "[this was a dry run; your system is unchanged]"
+  if "$DRYRUN"; then
+    echo "[this was a dry run; your system is unchanged]"
+  fi
 }
 
 # Start running things from a call at the end so if this script is executed