More doc on env var for PowerShell #409

Job	Run time
docs_rs_check	1m 13s
build_dynamic_and_test_windows_pre	6m 13s
build_static_and_test_ubuntu (release/6.0, nightly, no valgrind)	5m 41s
rust_clippy_check_ubuntu	5m 41s
build_static_and_test_ubuntu (release/6.0, 1.70.0, no valgrind)	6m 21s
rust_clippy_check_windows	1m 55s
rustfmt_check (macos-latest)	10s
build_static_and_test_ubuntu (release/6.1, nightly, valgrind)	1h 11m 8s
rustfmt_check (windows-latest)	31s
build_static_and_test_ubuntu (release/6.1, nightly, no valgrind)	5m 59s
rustfmt_check (ubuntu-latest)	12s
build_static_and_test_ubuntu (release/6.1, 1.70.0, no valgrind)	5m 34s
build_dynamic_and_test_windows (nightly)	1m 51s
build_dynamic_and_test_windows (1.70.0)	1m 47s
	1h 54m 16s

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