Run cargo update before attempting to build Rust compute packages

[phamann]

TL;DR

Invokes cargo update before attempting to build Rust based compute packages via fastly compute build. This is considered "safe" as cargo update will only update dependencies within their semver constraint ranges. However, does go against our philosophy to not mutate the users external state implicitly.

Why?

We currently use the semver version of the fastly-sys Rust crate to indicate major and minor changes to the underlying C@E ABI, rather than the typical meaning of Rust-breaking changes in the package itself. This has come back to bite us, as even if we keep the fastly-sys API itself stable, it depends on other packages that need to advance in major version increments, which requires a major version increment on fastly-sys in turn. We've seen when upgrading versions the fastly-sys gets stuck on an older version and causes rustc compilation error messages for the user, which can be confusing and misleading.

Therefore, we've decided to force an update of dependencies (and their transient deps) via cargo update, as a temporary solution, until we can externally manage the min/max versions of fastly-sys via a CLI configuration API.