Don't use incremental compilation for build scripts

[Mark-Simulacrum]

Similar to #8500, but going a step further by saying that usually build scripts aren't being edited by users, and even when they are, are quick to compile regardless. Incremental compilation is a net-negative if you're only going to compile once (compare "incremental full" and "full" on https://perf.rust-lang.org/dashboard.html).

This was noted while trying to cut down benchmark times on perf.rust-lang.org.

[ehuss]

There was some previous discussion about this here: #6577 (comment)

[weihanglo]

FWIW, there are more relevant discussions happening in #10493.

[ehuss]

The cargo team talked about this (notes). It wasn't clear exactly what the tradeoffs would be for performance here. Our guess is that it could be a wash either way. Without performance numbers, it's a bit hard to make a decision. If someone wants to move this forward, it would be helpful to have some benchmarking done with and without the change so that we could evaluate it. Particularly along the axes of:

• Small and large build scripts.
• Iterative changes to a build script versus just building the package from scratch.

It may be difficult to tease out the different factors, but some data would be a start.