For the ‘why’, see the docstring next to $SSL_CERT_DIR. In later commits,
packages will be changed to use these variables and the variables will be
added to more packages.
* guix/search-paths.scm ($SSL_CERT_DIR, $SSL_CERT_FILE): New variables.
* doc/guix.texi (Search Paths): Document them.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>