Add comment explaining why cc does not rebuild on env PATH change #1946

Job	Run time
Test (stable)	1m 15s
Test (beta)	1m 14s
Test (nightly)	1m 17s
Test (linux32)	2m 9s
Test (macos)	2m 55s
Test (aarch64-macos)	1m 32s
Test (aarch64-ios)	59s
Test (win32)	2m 7s
Test (win64)	3m 4s
Test (mingw32)	2m 23s
Test (mingw64)	3m 49s
Test (windows-2019)	3m 7s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	1m 35s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	3m 49s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	4m 53s
Test build-std (aarch64-tvos)	1m 42s
Test build-std (aarch64-tvos-sim)	2m 7s
Test wasm (wasm32-unknown-unknown)	22s
Test build-std (x86_64-tvos)	1m 46s
Test CUDA support	32s
Test wasm32-wasip1-thread	28s
Clippy	16s
MSRV (ubuntu-latest)	25s
Rustfmt	15s
MSRV (windows-latest)	37s
semver-checks	14s
MSRV (macos-latest)	27s
	45m 19s

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