`pyo3-build-config` cleanups

In https://github.com/PyO3/pyo3/pull/4758 I fixed the urgent bug with PYO3_CONFIG_FILE changes not correctly triggering rebuilds.

I think we can take action to ensure we don't have a similar accident again. I was playing around with various ideas, I've seen two so far which I think make sense:

• A lot of pyo3-build-config APIs require being run in a build script. I wonder if we should have something like a BuildScriptContext struct which all those APIs take, and it can collect info about environment accesses so that we can both test them and emit to cargo reliably.
• We can add clippy configuration to ban direct environment variable access, as suggested by @alex .