Cache for incremental builds

[dakom]

Running multiple times from WAVS repo is very slow, even with no changes to code

[ismellike]

Incremental builds are not deterministic: https://rustc-dev-guide.rust-lang.org/queries/incremental-compilation-in-detail.html#a-question-of-stability-bridging-the-gap-between-compilation-sessions

I saw at least one case where we ran just wasi-build before publishing, and we failed on the checksum diff most likely because of incremental builds.

[dakom]

I think this should be optional then - for our needs on the wavs repo we care about speed and stripping personal information from builds, not necessarily verifying a .wasm against source. Nondeterminism in incremental builds is fine.

For use in the wild, you're 100% right, we should have an option to ensure it always does a clean build for verification purposes

(what we could then do is have CI build the .wasm as a clean build on the wavs repo and check that in, but local just wasi-build would still be fast... but that's s different discussion. Having this be optional would unlock that.)