[DEPENDS] Allow to limit the jobs when building packages with ninja

Summary:
Pass the JOBS variable to ninja to allow for limiting the number of
jobs when building the packages.

Test Plan:

cd depends
make build-osx # This target should be benefit the most from this diff

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4875