Make CLI build faster

[joaomoreno]

Not only is the CLI stage the current bottleneck for our build's first stage, it also keeps a lot of agents busy as it runs. We should look into speeding it up. An obvious first step is to cache the target folder... maybe even use something like sccache.

[connor4312]

sccache didn't end up being that useful. It seemed like maybe a 10% time reduction, but these are also pretty noisy, so it's hard to tell.

Instead I opted to only run a dev build with clippy for main's CI, which is much faster than a full optimized production build. Real production builds are unchanged, but I think these are a much lesser concern since they only run ~once a day.