refactor: make with_opt_nix_path() a util #904

Job	Run time
macos	3m 46s
minver	11s
macos-aarch64	1m 45s
linux_native_builds (x86_64-unknown-linux-gnu)	2m 32s
linux_native_builds (x86_64-unknown-linux-musl)	2m 25s
rust_stable	1m 13s
rustfmt	8s
tier3 (x86_64-unknown-openbsd)	1m 1s
redox	37s
cross (arm-unknown-linux-gnueabi)	2m 7s
cross_compiling (aarch64-linux-android)	27s
cross (armv7-unknown-linux-gnueabihf)	2m 10s
cross_compiling (arm-linux-androideabi)	22s
cross (i686-unknown-linux-gnu)	2m 0s
cross_compiling (armv7-linux-androideabi)	40s
cross (i686-unknown-linux-musl)	2m 10s
cross_compiling (i686-linux-android)	43s
cross (mips-unknown-linux-gnu)	1m 45s
cross_compiling (x86_64-linux-android)	25s
cross (mips64-unknown-linux-gnuabi64)	2m 14s
cross_compiling (arm-unknown-linux-musleabi)	49s
cross (mips64el-unknown-linux-gnuabi64)	1m 57s
cross_compiling (x86_64-unknown-fuchsia)	29s
cross (mipsel-unknown-linux-gnu)	2m 8s
cross_compiling (x86_64-unknown-illumos)	51s
cross (powerpc64le-unknown-linux-gnu)	1m 41s
cross_compiling (aarch64-apple-ios, true)	26s
cross_compiling (powerpc64-unknown-linux-gnu)	34s
cross_compiling (s390x-unknown-linux-gnu)	59s
cross_compiling (x86_64-unknown-linux-gnux32)	38s
cross_compiling (x86_64-unknown-netbsd)	37s
	39m 50s

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