sys-apps/guix: make ebuild FEATURES=network-sandbox friendly, bug #607988

sys-apps/guix: make ebuild FEATURES=network-sandbox friendly, bug #607988

In order to shrink tarball size guix does not ship
prebuilt guile for supported targets. Instead it
downloaded those binaries and verified hashes in
gnu/local.mk file.

Fix it by pulling all external dependencies into SRC_URI.

Found by Toralf Förster on a tinderbox with FEATURES=network-sandbox

While at it disabled parallel builds as it occasionally
causes guile to run bad code.

Reported-by: Toralf Förster
Bug: https://bugs.gentoo.org/607988

Package-Manager: Portage-2.3.3, Repoman-2.3.1