Support minimum_zig_version in build.zig.zon

[alexrp]

It would be nice if there was a way to get the action to use minimum_zig_version from build.zig.zon.

Because I strongly favor predictable and reproducible behavior by default in CI scripts, I would argue that, if version is not given and a build.zig.zon with this field is present, the action should use it by default (this is how setup-dotnet works, for example).

If using minimum_zig_version as a default is not desirable for some reason, then an explicit gesture to opt in would be the next-best thing, e.g. version: build.zig.zon or something similar.