build(zig): use build root instead of cwd (tree-sitter#3944)

`std.fs.cwd` refers to the cwd of the build process, which is not the
root of tree-sitter when tree-sitter is used as a sub-module.

Co-authored-by: ObserverOfTime <[email protected]>

build.zig

@@ -99,7 +99,7 @@ fn wasmtimeDep(target: std.Target) []const u8 {
 fn findSourceFiles(b: *std.Build) ![]const []const u8 {
   var sources = std.ArrayList([]const u8).init(b.allocator);
 
-  var dir = try std.fs.cwd().openDir("lib/src", .{ .iterate = true });
+  var dir = try b.build_root.handle.openDir("lib/src", .{ .iterate = true });
   var iter = dir.iterate();
   defer dir.close();