Fix screencast on non-systemd distros.
On systemd distros, pipewire gets started as an user unit and is already available when kwin starts. On non-systemd distros (eg. Artix, Gentoo, Slackware), pipewire gets usually started using xdg-autostart, after kwin. MR !4794 (merged) moved the pipewire core initialization at the start of the screencast plugin, when pipewire is usually not yet available on non-systemd distros. Since the pipewire core is not recreated for every stream, this completely broke screencast with the infamous "Failed to connect PipeWire context" message. Instead of reverting MR !4794 (merged), this MR moves the pipewire core initialization at the first screencast request.