Adds comment to regress-builder about its current brokenness.

This is the last remaining included Python program that isn't
compatible with Python 3, and it doesn't make sense to fix that
unless someone is motivated to fix its general outdatedness.

TESTED:
No.

Signed-off-by: Gary E. Miller <[email protected]>

devtools/regress-builder

@@ -16,6 +16,14 @@ to seed from the system time.
 If no options are given, the script performs a sequential test of all
 possibilities.  This requires time on the order of 2 ** n where n is
 the count of options.
+
+***IMPORTANT***:
+Not only has this program not been updated for Python 3, but it hasn't been
+touched in such a long time that it still expects to use make rather than
+SCons to do builds (and probably has outdated notions of the drivers as well).
+Although updating it for Python 3 doesn't look too difficult, it doesn't make
+sense to do so without first restoring it to correct operation with Python 2,
+since the former fixes would otherwise be untestable.
 """
 import os, sys, random, getopt, time