systemd: fork pipewire to build w/ systemd support (MR 5142)

Unfortunately there are some code paths that are required to support
socket activation, and they are only enabled at build time with
-Dsystemd=enabled. On the bright side, service/socket unit files are
included...

Also note that I disabled test on armv7 since they fail in our CI using
qemu on x86_64. They pass in Alpine on native armv7.