capnproto: fix fibers on static builds•••This was broken (https://git.lix.systems/lix-project/lix/issues/955) and
it's just us not using libucontext (and the libucontext package being
broken).
Fibers should basically be supported on all platforms that matter and
it's mostly just some packaging mishaps that stop them from working, so
we want to turn on the requirement by default so that it works.
(cherry picked from commit 784852621a62a165a11752c09611505969a758bb)
origin/backport-432323-to-release-25.05