[phb:security] Fix vuln.xml testing procedure

Summary:
In r562203
and r562204 the vuln.xml file was splitted by year.

As stated in the commit message, pkg(8) audit does not support entities and hence,
we need to use the vuln-flat.xml file to test changes to the port.

Test Plan:

• Try something like this:

$ pkg audit -f ./vuln.xml gitea-1.13.4
pkg: Syntax error while parsing vulnxml
pkg: cannot process vulnxml

and then:

$ pkg audit -f ./vuln-flat.xml gitea-1.13.4
0 problem(s) in 0 installed package(s) found.

After the patch:

• igor clean
• The documentation is rendered properly.

Reviewers: 0mp, gbe

Differential Revision: https://reviews.freebsd.org/D29219