specs: don't rely on `_G.arg[-1]:match "/lua[0-9.]*$"`

When `specs/optparse_spec.yaml` came over from Specl, I forgot to
upgrade the direct `hell.spawn` invocations to nicely abstracted
`spec_helper.lua:luaproc` calls.
* specs/optparse_spec.yaml (parser): Replace hell.spawn calls
with luaproc calls, so that Lua interpreter is set correctly.

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