fix finding toolchains when invoked by msbuild #34

Job	Run time
Clippy	14s
Test CUDA support	25s
Test build-std (aarch64-tvos)	1m 43s
Test wasm (wasm32-unknown-unknown)	21s
Test build-std (aarch64-tvos-sim)	1m 55s
MSRV (ubuntu-latest)	25s
Test build-std (x86_64-tvos)	1m 48s
Rustfmt	14s
MSRV (windows-latest)	39s
semver-checks	17s
MSRV (macos-latest)	39s
Test (stable)	35s
Test (beta)	32s
Test (nightly)	1m 22s
Test (linux32)	1m 37s
Test (macos)	3m 18s
Test (aarch64-macos)	1m 15s
Test (aarch64-ios)	37s
Test (win32)	1m 30s
Test (win64)	1m 42s
Test (mingw32)	1m 42s
Test (mingw64)	1m 40s
Test (windows-2019)	2m 5s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	3m 1s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	1m 55s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	3m 14s
	34m 45s

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