Fix "FileExistsError" when "platformio ci" command is used in pair with "--keep-build-dir" option

platformio/commands/ci.py

@@ -55,7 +55,6 @@ def validate_path(ctx, param, value):  # pylint: disable=unused-argument
     "--build-dir",
     default=mkdtemp,
     type=click.Path(
-        exists=True,
         file_okay=False,
         dir_okay=True,
         writable=True,
@@ -134,7 +133,8 @@ def _copy_contents(dst_dir, contents):
     if dst_dir_name == "src" and len(items['dirs']) == 1:
         copytree(list(items['dirs']).pop(), dst_dir, symlinks=True)
     else:
-        makedirs(dst_dir)
+        if not isdir(dst_dir):
+            makedirs(dst_dir)
         for d in items['dirs']:
             copytree(d, join(dst_dir, basename(d)), symlinks=True)