make the cwd of RunStep take a Cache.Directory

[andrewrk]

Extracted from #14647.

zig/lib/std/Build/RunStep.zig

Lines 24 to 27 in b4d58e9

	/// Set this to modify the current working directory
	/// TODO change this to a Build.Cache.Directory to better integrate with
	/// future child process cwd API.
	cwd: ?[]const u8,

zig/lib/std/Build/RunStep.zig

Lines 858 to 863 in b4d58e9

	if (self.cwd) |cwd| {
	child.cwd = b.pathFromRoot(cwd);
	} else {
	child.cwd = b.build_root.path;
	child.cwd_dir = b.build_root.handle;
	}

zig/lib/std/child_process.zig

Lines 58 to 63 in b4d58e9

	/// Set to change the current working directory when spawning the child process.
	cwd: ?[]const u8,
	/// Set to change the current working directory when spawning the child process.
	/// This is not yet implemented for Windows. See https://github.com/ziglang/zig/issues/5190
	/// Once that is done, `cwd` will be deprecated in favor of this field.
	cwd_dir: ?fs.Dir = null,