This also switches to use guile-next, which includes important fixes to
invalid unicode handling with suspendable ports, which is necessary for the
agent.
* gnu/packages/package-management.scm (guix-build-coordinator): Update to
0-94.c4c4f5a.
* gnu/services/guix.scm: Use guile-next for the coordinator service.
Change-Id: I853539a612fd0b41a6c63069c6291b3f42e5cdee