CircleCI: reduce ASAN parallelism

[oschaaf]

Another stab at #272
Since the last Envoy update the ASAN build runs out of memory more often then not.

I noticed that the earlier attempt at reducing NUM_CPUS/--jobs didn't yield expected
results in CI, as bazel would still be running 8 jobs in parallel.

Upon investigation, I noticed that my earlier attempt had issues:

• it set the wrong variable
• upon fixing that, the script would end up handing two --jobs=x arguments to bazel

The approach here sets NUM_CPUS early on in the script, and doesn't attempt to mess
with the build flags later on.

Hopefully fixes #272

Signed-off-by: Otto van der Schaaf oschaaf@we-amp.com

[oschaaf]

Yeah, checking the CI build logs, bazel seems to run the expected amount of parallel actions (5) for ASAN. Marking this is ready for review.