msvc: allow finding tools from path if VisualStudioDir is set #31

Job	Run time
Test CUDA support	44s
Clippy	15s
Test build-std (aarch64-tvos)	1m 44s
Test wasm (wasm32-unknown-unknown)	14s
Test build-std (aarch64-tvos-sim)	1m 47s
Test build-std (x86_64-tvos)	1m 51s
MSRV (ubuntu-latest)	2m 58s
semver-checks	2m 33s
MSRV (windows-latest)	3m 18s
Rustfmt	7s
MSRV (macos-latest)	3m 2s
Test (stable)	1m 23s
Test (beta)	1m 29s
Test (nightly)	1m 26s
Test (linux32)	1m 30s
Test (macos)	3m 11s
Test (aarch64-macos)	1m 8s
Test (aarch64-ios)	1m 31s
Test (win32)	3m 4s
Test (win64)	2m 37s
Test (mingw32)	3m 2s
Test (mingw64)	3m 0s
Test (windows-2019)	2m 52s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	2m 59s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	3m 56s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	3m 34s
	55m 15s

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