Allow specify cache key when caching

[dexterorion]

Hey there. Hope everyone is well and safe.

We've been using reviewdog in our company, and it has been working really great.

But we are facing a challenge. We are using pull_request_target for our specific purpose, which leads to some concurrency problems when saving / restoring caches.

Because pull_request_target runs under base branch, when we have multiple PRs pointing to base branch, it causes some conflicts.

The way we see it might solve our specific problem is allowing us to control which is the cache key value, instead of using the generated one: https://github.com/reviewdog/action-golangci-lint/blob/master/src/cache.ts#L17

I am wondering if that is a desired feature for reviewdog action-golangci-lint.

If it is, I can test a solution within my company and provide further pull request here.