MacOS: xsltproc fails with "warning: failed to load external entity"

Hi hackers,

I'm having some difficulties building the documentation on MacOS.

I'm using ./full-build.sh script from [1] repository. It worked just
fine for many years but since recently it started to fail like this:

```
/usr/bin/xsltproc --path . --stringparam pg.version '16devel'
/Users/eax/projects/c/pgscripts/../postgresql/doc/src/sgml/stylesheet.xsl
postgres-full.xml
error : Unknown IO error
warning: failed to load external entity
"http://docbook.sourceforge.net/release/xsl/current/xhtml/chunk.xsl"
compilation error: file
/Users/eax/projects/c/pgscripts/../postgresql/doc/src/sgml/stylesheet.xsl
line 6 element import
xsl:import : unable to load
http://docbook.sourceforge.net/release/xsl/current/xhtml/chunk.xsl
error : Unknown IO error
/Users/eax/projects/c/postgresql/doc/src/sgml/stylesheet-html-common.xsl:4:
warning: failed to load external entity
"http://docbook.sourceforge.net/release/xsl/current/common/entities.ent"
%common.entities;
^
Entity: line 1:
%common.entities;
^
[...]

```

This is not a network problem. I can download chunk.xsl with wget and
also build the documentation on my Linux laptop.

I've tried `brew reinstall` and also:

```
./configure ... XMLLINT="xmllint --nonet" XSLTPROC="xsltproc --nonet"
```

... as suggested by the documentation [2] but it didn't change anything.

I checked the archive of pgsql-hackers@ but was unable to find
anything relevant.

I'm using MacOS Monterey 12.6.2.

```
$ brew info docbook
==> docbook: stable 5.1 (bottled)
...
$ brew info docbook-xsl
==> docbook-xsl: stable 1.79.2 (bottled)
...
```

At this point I could use a friendly piece of advice from the community.

[1]: https://github.com/afiskon/pgscripts/
[2]: https://www.postgresql.org/docs/15/docguide-toolsets.html

--
Best regards,
Aleksander Alekseev