services: Do not use symbolic links in PulseAudio variables.

This addresses <https://bugs.gnu.org/40837> by making these configuration
files more easily accessible within the WebKitGTK sandbox.

* gnu/services/sound.scm (pulseaudio-environment): Move below
PULSEAUDIO-CONF-ENTRY.  Instantiate PULSE_CONFIG and PULSE_CLIENTCONFIG
entries directly instead of referring to /etc/pulse.
(pulseaudio-etc): Do not create /etc/pulse/client.conf and /etc/pulse/daemon.conf.