Add `getOptionalEnvVar` helper #6130

Job	Run time
Ruby analysis (ubuntu-latest, linked)	48s
Ruby analysis (macos-latest, default)	1m 7s
Ruby analysis (ubuntu-latest, default)	47s
Ruby analysis (macos-latest, linked)	56s
Ruby analysis (ubuntu-latest, nightly-latest)	52s
Ruby analysis (macos-latest, nightly-latest)	1m 11s
	5m 41s

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