refactor: change posix_spawn path arg to use NixPath #1798

Job	Run time
macos-aarch64	2m 35s
linux_native_builds (x86_64-unknown-linux-gnu)	3m 25s
linux_native_builds (x86_64-unknown-linux-musl)	3m 23s
macos	4m 53s
minver	12s
rust_stable	1m 25s
rustfmt	14s
solaris (x86_64-pc-solaris)	4m 6s
tier3 (x86_64-unknown-dragonfly)	1m 24s
tier3 (x86_64-unknown-openbsd)	1m 2s
tier3 (x86_64-unknown-haiku)	56s
tier3 (armv7-unknown-linux-uclibceabihf)	1m 12s
tier3 (i686-unknown-hurd-gnu)	59s
redox	40s
cross (arm-unknown-linux-gnueabi)	2m 0s
cross_compiling (aarch64-linux-android)	38s
cross (armv7-unknown-linux-gnueabihf)	1m 54s
cross_compiling (arm-linux-androideabi)	36s
cross (i686-unknown-linux-gnu)	1m 49s
cross_compiling (armv7-linux-androideabi)	37s
cross (i686-unknown-linux-musl)	2m 2s
cross_compiling (i686-linux-android)	36s
cross (powerpc64le-unknown-linux-gnu)	1m 45s
cross_compiling (x86_64-linux-android)	38s
cross_compiling (arm-unknown-linux-musleabi)	46s
cross (loongarch64-unknown-linux-gnu)	1m 45s
cross_compiling (x86_64-unknown-fuchsia)	30s
cross_compiling (x86_64-unknown-illumos)	29s
cross_compiling (aarch64-apple-ios, true)	31s
cross_compiling (powerpc64-unknown-linux-gnu)	39s
cross_compiling (s390x-unknown-linux-gnu)	38s
cross_compiling (x86_64-unknown-linux-gnux32)	44s
cross_compiling (x86_64-unknown-netbsd)	35s
	45m 38s

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