bug#40806: xorg-configuration->file doesn't honor xorg-configuration-server

[Caleb Ristvedt]

Suppose that one specifies a custom xorg-server package to use in an
<xorg-configuration> record, like so:

(xorg-configuration
 (server xorg-server-for-debugging))

the correct bin/X will be run, but the wrong modules will be specified
in xserver.conf - the default xorg-server will always be used for
these. This is because in gnu/services/xorg.scm, in
xorg-configuration->file, we simply hard-code xorg-server to refer to
the one from (gnu packages xorg) and ignore the one specified in the
configuration entirely.

We shouldn't do this. It should be enough to just wrap the current
contents with

(let ((xorg-server (xorg-configuration-server config)))
  ...)

to make it work.

Then I can improve my xorg bug report :-)

- reepca