std.Build.Step.Run: convert relative paths to be relative to child cwd

Because any `LazyPath` might be resolved to a relative path, it's
incorrect to pass that directly to a child process whose cwd might
differ. Instead, if the child has an overriden cwd, we need to convert
such paths to be relative to the child cwd using `std.fs.path.relative`.
14e033ed95mlugg committed on 6/13/2025, 3:30:39 PMparent5c8b92d
1 file changedLine totals unavailable