Fix debian install path to actual path

__SHELLFU_MODNAME__ is not yet subject to ENV:PREFIX, so this part has
to be hard-coded.  (Anyway, Debian will want it to ne /usr.)

packaging/debian/install

@@ -1,2 +1,2 @@
 /usr/share/shellfu/include-bash/__SHELLFU_MODNAME__.sh
-__SHELLFU_LIBEXEC__/__SHELLFU_MODNAME__
+/usr/libexec/shellfu-bash-pxpath/pxpath