zig build: don't add --fork to build runner args

src/main.zig

@@ -5012,6 +5012,7 @@ fn cmdBuild(gpa: Allocator, arena: Allocator, io: Io, args: []const []const u8,
                         },
                         .failed = false,
                     });
+                    continue;
                 } else if (mem.eql(u8, arg, "--system")) {
                     if (i + 1 >= args.len) fatal("expected argument after '{s}'", .{arg});
                     i += 1;