[BREAKING] Provide runtime environment systemd services from munix

These services evolve as munix evolves, so they should not be part of
the system closures themselves. Mount them into /run/systemd instead.

(Yes, making /run/systemd/system a symlink to RO files is unfortunate,
 that could be changed in the future. FS prep code is annoying too..)

systemd/pipewire-bridge.service

@@ -0,0 +1,12 @@
+[Unit]
+Description=PipeWire VM bridge
+Requires=pipewire-bridge.socket
+
+[Service]
+Type=exec
+ExecStart=/opt/bin/muvm-pwbridge
+
+# Environment=RUST_LOG=debug
+# StandardError=tty
+# StandardOutput=tty
+# TTYPath=/dev/hvc0