child_process: add tracing channel for spawn #3395

Job	Run time
Build slim tarball	2m 20s
aarch64-linux: with shared libraries	35m 44s
x86_64-darwin: with shared libraries	1h 12m 38s
x86_64-linux: with shared libraries	54m 57s
aarch64-darwin: with shared libraries	46m 16s
	3h 31m 55s

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