Do not duplicate call to `getenv`. #807

Job	Run time
linux_intel (openssl-3.1.0)	5m 49s
linux_baseline	3m 51s
linux_intel (master)	6m 33s
Security checks	6m 55s
aarch64 cross-compilation	7m 6s
linux_baseline (-DOQS_KEM_ENCODERS=ON)	4m 8s
	34m 22s

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