Build specific commit for nvim nightly

Hi!

I maintain some neovim plugins and run some end-to-end tests in my github actions runs to try to make them easier to maintain. For this purpose, I try to be very strict about the neovim versions (both releases and nightly commits), so that the test runs are more reproducible. With many dependencies, the dependencies drifting can make maintenance a headache. 🙂

I recently implemented support in one of my projects for building+caching neovim's nightly version at a specific commit id (see the setup-neovim-nightly and download-neovim-nightly actions).

I was wondering if there is any interest to upstream this functionality to this action.

I took a quick look at the current implementation in this action, and it looks like the major differences are:

1. the latest neovim commit is always used, not possible to configure which commit to build
2. It looked like there is no caching. Perhaps the common case is to download the prebuilt nightly so there is no need to do it.

Not sure how well this would fit but figured it could be good to throw some ideas around at least 🙂

Start by reading the current action implementation and the linked setup-neovim-nightly and download-neovim-nightly action.yml files. Determine how a commit can be configured and how nightly artifacts can be cached across the supported platforms; done means the action can build and cache the requested commit reproducibly.