fix(ci): share GitHub runner caches between branches

[teor2345]

Motivation

Our Linux and macOS builds using GitHub runners are very slow the first time we open a PR on a new branch, because they aren't using any caches.

We need to build the Zcash parameter and Rust caches on the main branch, if we want them to be available to newly
created branches.

This should give us a massive performance improvement the first time we open a PR for a new branch.

Specifications

A workflow can access and restore a cache created in the current branch, the base branch (including base branches of forked repositories), or the default branch (usually main). For example, a cache created on the default branch would be accessible from any pull request. Also, if the branch feature-b has the base branch feature-a, a workflow triggered on feature-b would have access to caches created in the default branch (main), feature-a, and feature-b.

Access restrictions provide cache isolation and security by creating a logical boundary between different branches. For example, a cache created for the branch feature-a (with the base main) would not be accessible to a pull request for the branch feature-b (with the base main).

https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#restrictions-for-accessing-a-cache

Solution

• Run the CI and lint jobs on the main branch
• Cache the clippy job Rust dependencies

This is a quick fix, we can probably do better later.

Review

Anyone can review this PR.

I'd like @gustavovalverde to check that I got this right.

Reviewer Checklist

• CI passes
• Configs match specs

[gustavovalverde]

LGTM!