build: use spawn instead of spawnSync for build

[jkleinsc]

• Followup to build: use spawnSync instead of execFileSync #741
  That PR unfortunately did not solve the issue as we are still seeing this type of error in Windows builds:

3.91s Error: failed to load build.ninja: open obj\components\optimization_guide\core\features.ninja: The parameter is incorrect.

In theory its possible that spawnSync could cause file contention since it is synchronous, so this PR changes to instead use child_process.spawn to call ninja/autoninja and gn.