This is a workaround for pmb#2618, basically.
To build systemd with BPF support (so things like unpriv. nspawn work),
we have to install linux-stable-dev to get a special header(vmlinux.h).
This kernel (linux-stable) isn't build in Alpine for armhf. Normally
that's not a problem, we could just add linux-stable-dev conditionally
to the makedepends, right? No... not possible until pmb#2618 is fixed.
Given this arch is used by only 3 devices in testing (the rest are in
`downstream` or `archive`), I think it makes sense to disable systemd on
this arch until someone has the motivation to fix the pmb bug.
Part-of: https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/6637
[ci:skip-build]: already built successfully in CI