Fatalise warnings in regen/embed.pl when run in workflow?

https://github.com/Perl/perl5/pull/24361 accidentally introduced new `porting/regen.t` warnings by adding apidoc for macros that didn't have any. It would have been easier to spot these if the CI workflow had failed. Can we introduce an environment variable or other mechanism so that new warnings here cause the workflow to fail, but are non-fatal by default?