PARALLEL_MAKE default to core count

[mdavidsaver]

It looks like GHA changed their build environment at some point. As I read it, runners now get 4 cores.

python 3.4 added os.cpu_count(), which could provide a dynamic default.

ci-scripts/cue.py

Lines 154 to 156 in 4e4f33e

	ci['parallel_make'] = 2
	if 'PARALLEL_MAKE' in os.environ:
	ci['parallel_make'] = int(os.environ['PARALLEL_MAKE'])

[ralphlange]

Closed by #106.

[mdavidsaver]

A quick test with PVXS shows that ubuntu-latest does now build with -j4. Overall runtime seems to be about the same so far. No magic 50% speedup :(

[ralphlange]

We would probably need to optimize the layout of our source files. (Not suggesting this.)