Skip symlinks in _make_tree_writable()

Trying to chmod a symlink results in trying to chmod the file or
directory it points to, not the symlink itself which has no
permissions. Either a symlink points to within the tree we're making
writable in which case it'll be handled eventually by os.walk() or
it points outside of the tree we're making writable in which case
we don't want to touch it. Let's avoid touching files outside of the
tree by simply skipping symlinks in _make_tree_writable().

mesonbuild/utils/universal.py

@@ -1902,7 +1902,7 @@ def _make_tree_writable(topdir: T.Union[str, Path]) -> None:
         os.chmod(d, os.stat(d).st_mode | stat.S_IWRITE | stat.S_IREAD)
         for fname in files:
             fpath = os.path.join(d, fname)
-            if os.path.isfile(fpath):
+            if not os.path.islink(fpath) and os.path.isfile(fpath):
                 os.chmod(fpath, os.stat(fpath).st_mode | stat.S_IWRITE | stat.S_IREAD)