This oversubscribes our cores 2x, which testing shows we actually
run with at times: speedup is around a third on average for builds,
and a bit more than that for testing.
Also some light Bashification in ci/build.sh, for consistency.
Using AddressSanitizer/LeakSanitizer is better. It covers the full
unit/baseline test suite by default without requiring one to write
specific memory leak tests. It also covers other types of memory errors
besides just leaks.
