system: Use 'file-append' to denote file names.

system: Use 'file-append' to denote file names.

* gnu/services/avahi.scm, gnu/services/base.scm,
gnu/services/databases.scm, gnu/services/dbus.scm,
gnu/services/desktop.scm, gnu/services/dict.scm,
gnu/services/mail.scm, gnu/services/networking.scm,
gnu/services/sddm.scm, gnu/services/spice.scm,
gnu/services/ssh.scm, gnu/services/web.scm,
gnu/services/xorg.scm, gnu/system.scm: Replace the
 #~(string-append #$pkg "/bin/foo") idiom with
 (file-append pkg "/bin/foo").