Rollup merge of #108573 - kornelski:runtimeenvs, r=WaffleLapkin

Explain compile-time vs run-time difference in env!() error message

This PR is clarifying error message of `env!()` based on this user question: https://users.rust-lang.org/t/environment-variable-out-dir-is-undefined/90067

It makes it clear that `env!()` is for env variables defined at compile-time. There's special-case help text for common Cargo build script variables.

I've also rearranged the code to avoid allocating error message on the happy path when the env var is defined.

Author: matthiaskrgr