pyo3-build-config cleanups

[davidhewitt]

In #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 .

[ravenexp]

I will have some free time in the second half of December, and I'd like to try to do this, if it's not too late.

I'm still somewhat familiar with the pyo3-build-config code base from working on Windows cross-compilation support two years ago.

[davidhewitt]

Please go ahead, I think 0.24 will land sometime early in the new year so it would be great to see this happen in December :)