build: container: Add #:host-uids argument to call-with-container.

It's not always possible to map 65536 uids when creating a container as the
root user within another user namespace.  This is true when building Guix
within the build daemon's container.  By using a uid range of 1 by default,
even as the root user, the tests now pass.

* gnu/build/linux-container.scm (initialize-user-namespace, run-container):
  Add 'host-uids' argument.
  (call-with-container): Add #:host-uids keyword argument.
* tests/containers.scm ("container-excursion"): Update 'run-container' call.