[DEPENDS] Make the boost package build parallel

Summary:
The b2 boost build program runs with the available number of cores by
default (see the -j option
https://boostorg.github.io/build/manual/develop/index.html#bbv2.overview.invocation.options).
Passing it JOBS allow for the user to limit the number of core to be used if he wants to.

I didn't change the install -j4 parameter since I don't know where it
comes from, and I doubt it would make any difference since this step
will mostly copy files.

Test Plan:

cd depends
make

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4868