services: cups: Move SET-ENV to FILES-CONFIGURATION.

* gnu/services/cups.scm (cups-configuration): Move SET-ENV from here…
(files-configuration): …to here.
* doc/guix.texi (Printing Services): Adjust accordingly.

doc/guix.texi

@@ -14228,6 +14228,12 @@ programs.
 Defaults to @samp{"lp"}.
 @end deftypevr
+
+@deftypevr {@code{files-configuration} parameter} string set-env
+Set the specified environment variable to be passed to child processes.
+
+Defaults to @samp{"variable value"}.
+@end deftypevr
 @end deftypevr
 @deftypevr {@code{cups-configuration} parameter} access-log-level access-log-level
@@ -14755,12 +14761,6 @@ the output of the @code{uname} command.  @code{Full} reports @code{CUPS
 Defaults to @samp{Minimal}.
 @end deftypevr
-@deftypevr {@code{cups-configuration} parameter} string set-env
-Set the specified environment variable to be passed to child processes.
-
-Defaults to @samp{"variable value"}.
-@end deftypevr
-
 @deftypevr {@code{cups-configuration} parameter} multiline-string-list ssl-listen
 Listens on the specified interfaces for encrypted connections.  Valid
 values are of the form @var{address}:@var{port}, where @var{address} is

gnu/services/cups.scm

@@ -458,7 +458,10 @@ (define-configuration files-configuration
   (user
    (string "lp")
    "Specifies the user name or ID that is used when running external
-programs."))
+programs.")
+  (set-env
+   (string "variable value")
+   "Set the specified environment variable to be passed to child processes."))
 
 (define (serialize-files-configuration field-name val)
   #f)
@@ -811,9 +814,6 @@ (define-configuration cups-configuration
 reports @code{CUPS 2.0.0 (@var{uname})} where @var{uname} is the output of the
 @code{uname} command.  @code{Full} reports @code{CUPS 2.0.0 (@var{uname})
 IPP/2.0}.")
-  (set-env
-   (string "variable value")
-   "Set the specified environment variable to be passed to child processes.")
   (ssl-listen
    (multiline-string-list '())
    "Listens on the specified interfaces for encrypted connections.  Valid