Resolve formatter executable commands prior to calling them

[tylerjl]

Hi! Thanks for your work on apheleia, it's great.

An issue I've run into recently is that sometimes my remote executables aren't immediately in-$PATH: I need to resolve them via executable-find - which does find them - before invoking the process. This should be safe, given that the step is performed at a prior step. I'm overwriting the command variable since I infer that the executable name is being reused later when you're matching a callback, I think?

I mentioned that executable-find happens earlier in the call chain so I could maybe improve this by removing that and only resolving the executable once, but I wanted to check in with you to determine whether the overall problem and solution even make sense to somebody else.