Fix env duplication on merged-usr systems #1006

Job	Run time
Builds (windows-latest, x86_64-pc-windows-msvc)	3m 46s
Env Tests (ci-poetry-global, ubuntu-latest, x86_64-unknown-linux-musl)	2m 23s
Tests in Containers (ci-jupyter-container, ubuntu-latest, mcr.microsoft.com/devcontainers/univers...	4m 1s
Builds (windows-latest, aarch64-pc-windows-msvc)	4m 23s
Tests (windows-latest, x86_64-pc-windows-msvc, yes)	9m 47s
Env Tests (ci-poetry-project, ubuntu-latest, x86_64-unknown-linux-musl)	2m 11s
Tests in Containers (ci-homebrew-container, ubuntu-latest, homebrew/brew, x86_64-unknown-linux-musl)	3m 28s
Builds (ubuntu-latest, x86_64-unknown-linux-musl, yes)	1m 59s
Tests (ubuntu-latest, x86_64-unknown-linux-musl, yes)	7m 52s
Env Tests (ci-poetry-custom, ubuntu-latest, x86_64-unknown-linux-musl)	2m 17s
Builds (macos-latest, x86_64-apple-darwin, yes)	1m 40s
Env Tests (ci-poetry-global, windows-latest, x86_64-pc-windows-msvc)	4m 59s
Tests (macos-latest, x86_64-apple-darwin, yes)	9m 0s
Builds (macos-14, aarch64-apple-darwin, yes)	1m 47s
Env Tests (ci-poetry-project, windows-latest, x86_64-pc-windows-msvc)	5m 10s
Tests (macos-14, aarch64-apple-darwin, yes)	8m 28s
Env Tests (ci-poetry-custom, windows-latest, x86_64-pc-windows-msvc)	4m 22s
	1h 17m 33s

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