Documentation building fails on HTTPS redirect (again)

It seems like the issue discussed in [0] is back, but this time for XSL imports
via xsltproc. The http link now redirects with a 301 (since when I don't know,
but it worked recently):

$ curl -I http://docbook.sourceforge.net/release/xsl/current/xhtml/chunk.xsl
HTTP/1.1 301 Moved Permanently
Date: Fri, 30 Sep 2022 09:20:00 GMT
Connection: keep-alive
Cache-Control: max-age=3600
Expires: Fri, 30 Sep 2022 10:20:00 GMT
Location: https://docbook.sourceforge.net/release/xsl/current/xhtml/chunk.xsl
Content-Security-Policy: upgrade-insecure-requests
Server: cloudflare
CF-RAY: 752be1544eea0d2e-ARN
alt-svc: h3=":443"; ma=86400, h3-29=":443"; ma=86400

Changing the links in the documents to be https, and avoid the redirect,
doesn't help unfortunately since xsltproc can't download assets over https.
The lack of https support in libxml2 has been reported, and patches submitted,
a long time ago [1] but there is still a lack of https support. Looking around
for other mirrors I only managed to find cdn.dookbook.org, which just like
Sourceforge does a 301 redirect.

Installing the stylesheets locally as we document solves the issue of course,
but maybe it's time to move to using --nonet as we discussed in [0] and require
the stylesheets locally? It's a shame that casual contributions require a big
investment in installation, but it seems hard to get around.

--
Daniel Gustafsson https://vmware.com/

[0] https://www.postgresql.org/message-id/flat/E2EE6B76-2D96-408A-B961-CAE47D1A86F0(at)yesql(dot)se
[1] https://mail.gnome.org/archives/xml/2007-March/msg00087.html