release-small-linux.nix: a variant of release-small.nix that builds only on x86_64-linux
guile: disable test suite to fix build

The test suite requires the build machine to support IPv6:

  ERROR: socket.test: AF_INET6/SOCK_STREAM: bind - arguments: ((system-error "bind" "~A" ("Cannot assign requested address") (99)))
  ERROR: socket.test: AF_INET6/SOCK_STREAM: bind/sockaddr - arguments: ((system-error "bind" "~A" ("Cannot assign requested address") (99)))
perl: revert "remove one more unreliable test, fixes #454"

Commit 70f0d79d063c665a9fd46d5bd38391f940f0430f causes a massive rebuild
that I'd like to avoid until my GCC 4.8.x switch is done.