Fixes <https://issues.guix.gnu.org/66097>.
Previously, this test would actually need to access dist.libuv.org.
* tests/gnu-maintenance.scm ("rewrite-url, without to-version"): Change
URLs from https to http, and from libuv.org to libuv.example.org.
Parameterize ‘current-http-proxy’.
Reported-by: Janneke Nieuwenhuizen <janneke@gnu.org>