Do not duplicate call to `getenv`. #648

Job	Run time
cygwin (windows-2019, win64, -DOQS_ALGS_ENABLED=STD)	4m 57s
msvc (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A)	3m 5s
msvc (windows-2022, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A)	2m 25s
msvc_native (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	3m 53s
msvc_native (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	3m 29s
msvc_native (windows-2022, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	5m 15s
msvc_native (windows-2022, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	3m 25s
	26m 29s

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