Don't involve evloop after fork in `System::Process.spawn` (UNIX) #12269

Job	Run time
check_format	1m 39s
x86_64-gnu-test-preview_mt	5m 39s
deploy_api_docs	0s
x86_64-musl-test	35m 5s
x86_64-gnu-test (1.7.3)	36m 1s
x86_64-gnu-test (1.8.2)	35m 24s
x86_64-gnu-test (1.9.2)	32m 39s
x86_64-gnu-test (1.10.1)	33m 48s
x86_64-gnu-test (1.11.2)	36m 14s
x86_64-gnu-test (1.12.2)	34m 13s
x86_64-gnu-test (1.13.2)	35m 34s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	31m 14s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	38m 27s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	34m 30s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	35m 32s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	34m 19s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	34m 58s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	34m 51s
	8h 50m 7s

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