cert.pl no_port behavior via environment variable

[ericsmithkestrel]

I would like to be able to select "no_port" behavior in cert.pl (that is, not including the portcullis commands of included books in a given book's portcullis) globally, via an environment variable. We have a custom builder that (in one mode) calls cert.pl to do the certification. (In another mode, it does the certification itself and does not support the no_port behavior, so there is a mismatch.) Our builder could set the environment variable before calling cert.pl. I'd prefer not to have to add something to every book or every dir to select no_port. Thanks!