Make default version "latest"

[tfausak]

Instead of hard coding a default version that needs to be updated sometimes (#57), I think the default version should be "latest". That way users will get the latest version of HLint without anyone needing to change anything. And if users want to pin the current latest version at time of writing, they can specify the version input just like before.

I'd be happy to open a PR adding this functionality if it's desired. You can see how I do this with my cabal-gild-setup-action here: https://github.com/tfausak/cabal-gild-setup-action/blob/5aa7281b9e1d33818ed53230eef6f73dda3f89e1/index.js#L37

[mihaimaruseac]

That's a good idea. I haven't tested it before, but I think sensible defaults instead of hardcoded values are better.