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Fix env duplication on merged-usr systems (#200) #160

Job	Run time
Build (windows-latest, aarch64-pc-windows-msvc)	3m 16s
Build (macos-latest, x86_64-apple-darwin)	2m 38s
Build (macos-14, aarch64-apple-darwin)	2m 37s
Build (ubuntu-latest, x86_64-unknown-linux-musl)	1m 35s
Build (windows-latest, x86_64-pc-windows-msvc)	3m 35s
	13m 41s

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