feature: Rust compilation should use stable build directory

[emielbeinema]

Currently, compiling Rust scripts in windmill takes a very long time. One of the reasons is that the script and all its dependencies are recompiled (in 'release' mode) every time the script is built.

To combat long build times, Cargo, the Rust package manager, by default uses incremental compilation. It reuses compiled assets that have not changed, so it can skip their compilation. However, to be able to do that, Cargo needs the old assets to be in its build directory (at least some specific folders).

Currently, each build process of a Rust script in windmill runs in a fresh build directory, without assets from previous compilation. If windmill would run Rust builds of the same script by the same user in a build directory containing the assets from the previous build, that would speed up the compilation significantly.

In addition (independently from this feature request), using sccache as discussed in #4369 and as described in the Cargo documentation could speed up Rust compilation even across different scripts.