services: mpd: Do not hardcode environment variables.

Services should not expect for XDG_RUNTIME_DIR to be set.
Inferring from the past comment, this seemed to resolve an issue when
the service was launched with an _interactive_ user-account,
which tends to have this variable set by the login-manager.

This is not the case for system accounts and setting this variable
results in PulseAudio (launched by the same system user) failing to start
since it attempts to use a nonexistent directory.

Ideally, this service should have a home-service counterpart but the old
behavior can be emulated by setting the environment-variables field to:

(environment-variables
  (list
    (string-append "XDG_RUNTIME_DIR=/run/user/"
                   (number->string (passwd:uid (getpwnam "myuser"))))))

* gnu/services/audio.scm
(mpd-configuration)[environment-variables]: New field.
(mpd-shepherd-service)[start]: Use new field.
* doc/guix.texi (Audio Services)[Music Player Daemon]: Document it.

Signed-off-by: Liliana Marie Prikler <liliana.prikler@gmail.com>

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