Fixes a regression introduced in
938448bf40fc77092859352d2243e2d0c236375f 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.