Add `getOptionalEnvVar` helper #6132

Job	Run time
Ruby analysis (macos-latest, linked)	1m 13s
Ruby analysis (ubuntu-latest, linked)	49s
Ruby analysis (macos-latest, default)	1m 2s
Ruby analysis (ubuntu-latest, nightly-latest)	48s
Ruby analysis (macos-latest, nightly-latest)	1m 16s
Ruby analysis (ubuntu-latest, default)	46s
	5m 54s

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