Clear Yarn cache before `yarn install`

[m4theushw]

In https://github.com/mui/mui-x there's a dependency in package.json which is a git repo. In turns out that if a git repo has its history rewritten, e.g. by deleting a commit, Yarn will try to do a merge commit if it already has the repo cached and CodeSandbox CI will fail because it doesn't have the user and email to write the commit. Cleaning the cache before yarn install solves the problem locally but is there a way to do this inside CodeSandbox CI?