[cleanup] Minor fixes (See desc)

* [youtube] Fix `--youtube-skip-dash-manifest`
* [build] Use `$()` in `Makefile`. Closes #3684
* Fix bug in 385ffb467b
* Fix bug in 43d7f5a5d0
* [cleanup] Remove unnecessary `utf-8` from `str.encode`/`bytes.decode`
* [utils] LazyList: Expose unnecessarily "protected" attributes
and other minor cleanup

test/test_utils.py

@@ -1759,7 +1759,7 @@ Line 1
 
         def test(ll, idx, val, cache):
             self.assertEqual(ll[idx], val)
-            self.assertEqual(getattr(ll, '_LazyList__cache'), list(cache))
+            self.assertEqual(ll._cache, list(cache))
 
         ll = LazyList(range(10))
         test(ll, 0, 0, range(1))