Don't set CMAKE_SYSTEM_NAME when using the Visual Studio generator #163

Job	Run time
Rustfmt	13s
Test (ubuntu-latest, stable)	15s
Test (ubuntu-latest, beta)	30s
Test (ubuntu-latest, nightly)	19s
Test (macos-latest, stable)	47s
Test (macos-latest, beta)	38s
Test (macos-latest, nightly)	39s
Test (windows-latest, stable)	2m 2s
Test (windows-latest, beta)	2m 23s
Test (windows-latest, nightly)	1m 32s
Test Cross Compile - aarch64-apple-ios	42s
Test Cross Compile - aarch64-pc-windows-msvc	3m 48s
Test Cross Compile - aarch64-unknown-linux-gnu	51s
Test Cross Compile - arm-unknown-linux-gnueabihf	36s
Test Cross Compile - mips-unknown-linux-gnu	5s
Test Cross Compile - mips64-unknown-linux-gnuabi64	4s
Test Cross Compile - mips64el-unknown-linux-gnuabi64	4s
Test Cross Compile - mipsel-unknown-linux-gnu	6s
Test Cross Compile - powerpc-unknown-linux-gnu	38s
Test Cross Compile - s390x-unknown-linux-gnu	34s
Test Cross Compile - x86_64-unknown-linux-musl	44s
Test Cross Compile - aarch64-unknown-linux-musl	37s
Test Cross Compile - x86_64-pc-windows-gnu	1m 0s
Test Cross Compile - x86_64-unknown-freebsd	42s
Test Cross Compile - x86_64-unknown-netbsd	35s
Test Cross Compile - x86_64-sun-solaris	33s
Test Cross Compile - x86_64-unknown-illumos	36s
	21m 33s

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