Use std.Build.LazyPath in build.zig files

[toivosnare]

This way auto-generated files in the Zig build cache can be passed in. There is an automatic conversion from a string to LazyPath so this shouldn't break anything.

[Ivan-Velickovic]

Makes sense to use LazyPath now, I think when I originally made these it wasn't an option.

One comment I have is about the calls to getPath3, is that the only way to get the actual path of a LazyPath? The Zig stdlib docs say only to use that during the make phase.