bug#38254: Download code should honor /etc/ssl/certs/*.crt

[Ludovic Courtès]

Hello,

Some distros such as CentOS 7 with its ‘ca-certificates’ package provide
nothing but a certificate bundle in /etc/ssl:

--8<---------------cut here---------------start------------->8---
$ ls -l /etc/ssl/certs/
total 12
lrwxrwxrwx. 1 root root   49  8 nov.  16:44 ca-bundle.crt -> 
/etc/pki/ca-trust/extracted/pem/tls-ca-bundle.pem
lrwxrwxrwx. 1 root root   55  8 nov.  16:44 ca-bundle.trust.crt -> 
/etc/pki/ca-trust/extracted/openssl/ca-bundle.trust.crt
-rwxr-xr-x. 1 root root  610 30 oct.   2018 make-dummy-cert
-rw-r--r--. 1 root root 2516 30 oct.   2018 Makefile
-rwxr-xr-x. 1 root root  829 30 oct.   2018 renew-dummy-cert
--8<---------------cut here---------------end--------------->8---

As of commit 9c9982dc0c8c38ce3821b154b7e92509c1564317, ‘guix download’ &
co. (anything that relies on (guix build download)) fail because they
looks for /etc/ssl/certs/*.pem by default and there’s no such file.

Thanks,
Ludo’.