Zig should be more tolerant against invalid cache dir

[plajjan]

I've stumbled upon this numerous times. I usually just clear the cache and start over to fix it, but I think there's a zig bug here somehow. It's probably in conjunction with something else not working properly but nonetheless, the end result is failure, which is bad.

I suppose Zig has some cache manifest that says "this file should be in the cache" but for some reason it turns out to not be in the cache and then things just fail. I think that Zig could be more tolerant - simply check if the file actually exists on disk and not just in the manifest?