Enable `LIBOQS_DIR` as cache entry and environment variable. #260

Job	Run time
msvc (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A-masm)	1m 27s
cygwin (windows-2019, win64, -DOQS_ALGS_ENABLED=STD)	4m 53s
msvc_native (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	4m 3s
msvc_native (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	3m 59s
msvc_native (windows-2022, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	3m 58s
msvc_native (windows-2022, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	4m 13s
	22m 33s

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