bug#36811: Guix fails to build with libgc 8.0.4

[Jonathan Brielmaier]

Just for the record: Ludo added libgc-8 in
69961ac3b9db2dbb99abb5c45ff6be3a9c01b5de on master. It's not yet used,
so we need to replace libgc-7.6 with it and see how it goes...