fix: always write slash paths to RECORD file #706

Job	Run time
macos-aarch64	10m 6s
sdist	9m 18s
macos-x86_64	12m 0s
linux (x86_64-unknown-linux-gnu)	7m 37s
windows (x86_64-pc-windows-msvc, x64)	14m 15s
linux (i686-unknown-linux-gnu)	11m 8s
windows (i686-pc-windows-msvc, x86)	14m 50s
linux-arm (aarch64-unknown-linux-gnu, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	10m 37s
linux-powerpc (powerpc64le-unknown-linux-gnu, ppc64le, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	10m 53s
linux-arm (armv7-unknown-linux-gnueabihf, armv7)	10m 21s
linux-powerpc (powerpc64-unknown-linux-gnu, ppc64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	10m 35s
linux-arm (arm-unknown-linux-musleabihf, arm)	10m 13s
linux-s390x (s390x-unknown-linux-gnu, s390x)	14m 29s
musllinux (x86_64-unknown-linux-musl)	9m 58s
musllinux (i686-unknown-linux-musl)	10m 2s
musllinux-cross (aarch64-unknown-linux-musl, aarch64, -e JEMALLOC_SYS_WITH_LG_PAGE=16)	10m 58s
musllinux-cross (armv7-unknown-linux-musleabihf, armv7)	9m 58s
	3h 7m 18s

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