Limit cts-api-coverage parallelism to 8

cts-api-coverage is getting OOM killed in presubmit, likely due to
spawning a copy of dexdeps in parallel for every cpu in the system.
Limit it to 8 parallel subprocesses when running inside the build,
as other processes are likely already using cpu and memory.

Bug: 252817769
Test: m out/host/linux-x86/cts-api-coverage/system-api-coverage.html
Change-Id: Iaae6561ac173d32c1b5af895cab76f3f898da4bb