shepherd: 'fork+exec-command/container' always returns a PID.

Fixes a regression introduced in
938448bf40 whereby
'fork+exec-command/container' would return #t, then used as the running
value of the 'guix-daemon' service in the installer.  Upon installation
completion, stopping the 'guix-daemon' service would fail with
wrong-type-arg because that #t would be passed to the 'stop' method in
lieu of a PID.

* gnu/build/shepherd.scm (fork+exec-command/container): Return a PID
rather than #t.

gnu/build/shepherd.scm

@@ -240,7 +240,7 @@ on Hurd systems for instance, fallback to direct forking."
                      (apply exec-command* command (strip-pid args)))
                    (lambda ()
                      (primitive-_exit 127))))
-              (pid #t))))
+              (pid pid))))               ;XXX: assuming the same PID namespace
         (apply fork+exec-command command (strip-pid args)))))
 
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