pipewire.conf.in: use absolute path to pipewire-media-session
So pipewire daemon is able to find pipewire-media-session from the location it's installed to, even when it's not in $PATH. For the `pw-ininstalled.sh` and `make run` invocations, provide a `src/daemon/pipewire-uninstalled.conf` that still uses the relative path, but isn't installed.