Add `getOptionalEnvVar` helper #8194

Job	Run time
Multi-language repository (ubuntu-latest, stable-v2.17.6)	4m 27s
Multi-language repository (macos-latest, stable-v2.18.4)	13m 21s
Multi-language repository (macos-latest, stable-v2.19.4)	12m 27s
Multi-language repository (macos-latest, stable-v2.20.7)	10m 26s
Multi-language repository (macos-latest, stable-v2.21.4)	11m 53s
Multi-language repository (ubuntu-latest, stable-v2.18.4)	4m 13s
Multi-language repository (ubuntu-latest, stable-v2.19.4)	4m 53s
Multi-language repository (ubuntu-latest, nightly-latest)	4m 10s
Multi-language repository (macos-latest, stable-v2.17.6)	11m 57s
Multi-language repository (macos-latest, default)	14m 11s
Multi-language repository (ubuntu-latest, stable-v2.22.4)	4m 29s
Multi-language repository (ubuntu-latest, stable-v2.21.4)	4m 22s
Multi-language repository (ubuntu-latest, linked)	4m 5s
Multi-language repository (ubuntu-latest, default)	4m 17s
Multi-language repository (macos-latest, stable-v2.22.4)	18m 16s
Multi-language repository (macos-latest, nightly-latest)	18m 27s
Multi-language repository (ubuntu-latest, stable-v2.20.7)	4m 53s
Multi-language repository (macos-latest, linked)	15m 26s
	2h 46m 13s

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