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I looked into this last year and at the time it wasn't supported. Can't say for sure that's still the case but I did a quick search and couldn't find anything new

I guess what you are after is an <https://www.pulumi.com/docs/languages-sdks/yaml/yaml-language-reference/#expressions|expression> to read an environment variable. Can you please file an issue here?
<https://github.com/pulumi/pulumi-yaml/issues>

Done <https://github.com/pulumi/pulumi-yaml/issues/545>