gnumeric: fix wrapping

Incorporate wrapGAppsHook so that all gnumeric binaries are wrapped,
following the convention used by many gnome applications.

This addresses two issues:

1.  The packaged ssconvert, ssdiff, ssgrep, and ssindex executables
    in bin are not currently wrapped so some expected environment
    variables including XDG_DATA_DIRS and GIO_EXTRA_MODULES are not
    set. The result is many warnings on stderr when running these
    commands, e.g.

    ==================================================================
    CRITICAL **:...go_conf_add_monitor: assertion 'node || key' failed
    CRITICAL **:...go_conf_get_node: assertion 'parent || key' failed
    WARNING **:...unknown GOConfMonitor id.
    ==================================================================

2.  None of the binaries, including gnumeric, currently wrap the
    environment variable GDK_PIXBUF_MODULE_FILE. This can cause
    segfaults if an incompatible GDK_PIXBUF_MODULE_FILE is already set
    in the environment (e.g. by plasma5). This could be encountered
    running a nixos pre-19.03 gnumeric binary from a nixos 18.09 KDE
    session.