Add yarn install caching

[toninorair]

An attempt to speed up build and migrations

[toninorair]

Doesn't seem like this is speeding up anything? If it does though, let's merge!

It should, most steps even inside this job use cached dependencies like here https://github.com/compound-finance/comet/runs/6942130256?check_suite_focus=true

Also, it should be useful for the deployment step when the same job can be run multiple times due to rate limiting.

[jflatow]

Doesn't seem like this is speeding up anything? If it does though, let's merge!

It should, most steps even inside this job use cached dependencies like here https://github.com/compound-finance/comet/runs/6942130256?check_suite_focus=true

Also, it should be useful for the deployment step when the same job can be run multiple times due to rate limiting.

But just empirically, compare to e.g. https://github.com/compound-finance/comet/actions/runs/2551760112 it took 6m with the caching and 5m without?