build-cmssw-ib-with-patch: do not use all cores on ARMv7 machines

Most of Jenkins jobs for armv7hl are failing due to channel temrination.
This most likely happens because CMSSW compilation is memory bound and
sometimes we start heavily swapping. Minimize memory preasure by using
3 threads instead of 4.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>