Clarify how environment variables should be separated in `linux_env_vars` and `windows_env_vars` #109

Job	Run time
Test without Docker / Windows (5.9 - windows-2022)	3m 55s
Test with Docker / Linux (5.8 - jammy)	31s
Test with Docker / Windows (5.9 - windows-2022)	9m 43s
Test without Docker / Windows (6.0 - windows-2022)	3m 56s
Test without Docker / Linux (${{ matrix.swift_version }} - ${{ matrix.os_version }})	0s
Test with Docker / Linux (5.9 - jammy)	27s
Test with Docker / Windows (6.0 - windows-2022)	6m 23s
Test without Docker / Windows (nightly - windows-2022)	4m 34s
Test with Docker / Linux (5.10 - jammy)	27s
Test without Docker / Windows (nightly-6.1 - windows-2022)	4m 37s
Test with Docker / Windows (nightly - windows-2019)	10m 6s
Test with Docker / Linux (6.0 - jammy)	32s
Test with Docker / Linux (nightly-main - jammy)	33s
Test with Docker / Windows (nightly-6.1 - windows-2019)	9m 28s
Test with Docker / Linux (nightly-6.1 - jammy)	33s
Soundness / API breakage check	0s
Soundness / Broken symlinks check	4s
Soundness / Documentation check	35s
Soundness / License headers check	3s
Soundness / Unacceptable language check	3s
Soundness / Format check	0s
Soundness / Python lint check	5s
Soundness / Shell check	31s
Soundness / YAML lint check	3s
	57m 9s

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