plv8: unbreak build, bump to version 2.3.8

ee58a5b3

 broke the plv8 build because it
upgraded the v8_6_x expression everywhere to the 6.9 branch, which came
with API changes. Notably, it seems plv8 only supports up-to v8 6.4.x at
this time.

This keeps a copy of the plv8_6_x expression inside the same directory
as the other v8 versions (so patches, etc are easy to apply), but it is
not exposed to the top-level of all-packages.nix.

Signed-off-by: Austin Seipp <aseipp@pobox.com>