Fix flatpak build: --socket=pipewire is not a valid finish-arg

flatpak build-finish rejects it outright ("Unknown socket type
pipewire, valid types are: x11, wayland, pulseaudio, session-bus,
system-bus, fallback-x11, ssh-auth, pcsc, cups, gpg-agent,
inherit-wayland-socket"). Direct PipeWire socket access is granted via
--filesystem=xdg-run/pipewire-0 instead (same approach other
sandboxed apps needing raw PipeWire, e.g. OBS Studio's flatpak, use).

fluxer_desktop/electron-builder.config.cjs

@@ -1279,7 +1279,7 @@ module.exports = {
 			'--device=all',
 			// Audio output + WebRTC screen/mic capture via PipeWire
 			'--socket=pulseaudio',
-			'--socket=pipewire',
+			'--filesystem=xdg-run/pipewire-0',
 			'--filesystem=home',
 			'--share=network',
 			// System notifications