build: run test-hash-seed at the end of test-v8

The v8 and test-hash-seed targets cannot be run in parallel because they
need different copies of the deps/v8 directory.

Ref: https://github.com/nodejs/node/pull/14004#issuecomment-314774773
Backport-PR-URL: https://github.com/nodejs/node/pull/15562
PR-URL: https://github.com/nodejs/node/pull/14219


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>