gnu: glib-networking: Prefer 'SSL_CERT_FILE' as TLS CA file.

This is an improvement over commit af6100f5.

* gnu/packages/gnome.scm (glib-networking)[source](patches): New field.
  [arguments]: Pass '/etc/ssl/certs/ca-certificates.crt' to configure.
  Set 'SSL_CERT_FILE' to '/dev/null' in 'use-empty-ssl-cert-file' phase.
  (libsoup)[arguments]: Set 'SSL_CERT_FILE' to '/dev/null' in
  'pre-check' phase.
* gnu/packages/patches/glib-networking-ssl-cert-file.patch: New patch.
* gnu-system.am (dist_patch_DATA): Add it.