child_process: add tracing channel for spawn #3398

Job	Run time
Build slim tarball	2m 27s
aarch64-linux: with shared libraries	34m 32s
x86_64-darwin: with shared libraries	52m 52s
aarch64-darwin: with shared libraries	39m 36s
x86_64-linux: with shared libraries	44m 23s
	2h 53m 50s

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