cmd/api: limit concurrent 'go list' calls to GOMAXPROCS
Each invocation of 'go list' may consume a significant quantity of
system resources, including buffers for reading files and RAM for the
runtime's memory footprint.
Very small builders may even hit swap as a result of that load,
further exacerbating resource contention.
To avoid overloading small builders, restrict 'go list' calls to
runtime.GOMAXPROCS as it is set at the first call to loadImports.
This also somewhat improves running time even on larger machines: on
my workstation, this change reduces the wall time for 'go test
cmd/api' by around 100ms.