Add `getOptionalEnvVar` helper #8193

Job	Run time
Split workflow (ubuntu-latest, default)	1m 4s
Split workflow (macos-latest, default)	1m 41s
Split workflow (ubuntu-latest, linked)	1m 22s
Split workflow (macos-latest, nightly-latest)	1m 54s
Split workflow (ubuntu-latest, nightly-latest)	1m 10s
Split workflow (macos-latest, linked)	1m 13s
	8m 24s

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