specl/shell: manually pass package.path in remaining hell.spawn(specl) specs.

* specs/custom_formatters_spec.yaml (formatter): In case we're
using modules not on the system Lua package path, make sure to
manually pass our package.path to inferior specl spawn.
* specs/specl_spec.yaml: Likewise.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>