cmd/api: run half as many go list calls in parallel

We currently run one 'go list' invocation per GOMAXPROC. Since the go
command uses memory and has its own internal parallelism, that's
unlikely to be an efficient use of resources. Run half as many. I
suspect that's still too many but this should fix our OOMs.

For #49957.

Change-Id: Id06b6e0f0d96387a2a050e400f38bde6ba71aa60
Reviewed-on: https://go-review.googlesource.com/c/go/+/370376


Trust: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>