bug#54102: SSL_CERT_DIR is not always unary

[David Arroyo]

The guix shell profile, at least on non-guix SD systems, contains the line

        export 
SSL_CERT_DIR="${GUIX_PROFILE:-/gnu/store/xxxx-profile}/etc/ssl/certs${SSL_CERT_DIR:+:}$SSL_CERT_DIR"

Since it prepends to the SSL_CERT_DIR variable, if a silly user were to 
accidentally source this file twice, say, to pick up changes they've made to a 
file that sources this file, the variable will contain duplicate paths.

However, several locations in the guix source assume SSL_CERT_DIR is a single 
directory. As an example, I ran into this issue when attempting to use `guix 
import opam -r faraday`:

        Starting download of /tmp/guix-file.XFPss4
        From https://github.com/inhabitedtype/faraday/archive/0.8.1.tar.gz...
        X.509 certificate of 'github.com' could not be verified:
          signer-not-found
          invalid

Running the command with `strace -f` showed that guix was attempting to open 
$SSL_CERT_DIR, rather than the first colon-delimited item in $SSL_CERT_DIR.

It might be better to clobber this variable in the guix shell profile, rather 
than render it unusable for some subcommands. If not that, then we should 
remove the assumption that it contains a single path element.

(apologies if this is a duplicate email; I sent this earlier before subscribing 
to the list)