Use `build` directory instead of `_build`. #394

Job	Run time
cygwin (windows-2019, win64, -DOQS_ALGS_ENABLED=STD)	3m 55s
msvc (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A)	2m 22s
msvc_native (windows-2022, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	2m 39s
msvc_native (windows-2022, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	3m 11s
	12m 7s

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