Revert "Use `build` directory instead of `_build`. (#314)" #417

Job	Run time
cygwin (windows-2019, win64, -DOQS_ALGS_ENABLED=STD)	31m 45s
msvc (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A)	9m 57s
msvc_native (windows-2022, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	11m 47s
msvc_native (windows-2022, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	11m 38s
	1h 5m 7s

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