feature request: support 'env' that propogates to dependents

we have a helper crate that does codegen.

the code in it acts as a library for build scripts, when run it needs protoc. We can add protoc t the data attribute, and then the actual build script inherits that.

But the path to protoc passed in as an env variable doesn't propogate.

We could capture that with `env!` and then set the variable to that when actually running, but that feels wrong...

Happy to add some mock code if this isn't clear.