umu_run: skip proton file check

umu/umu_run.py

@@ -387,7 +387,7 @@ def build_command(
     # Exit if the entry point is missing
     # The _v2-entry-point script and container framework tools are included in
     # the same image, so this can happen if the image failed to download
-    if not entry_point.is_file():
+    if env.get("UMU_NO_PROTON") != "1" and not entry_point.is_file():
         err: str = (
             f"_v2-entry-point (umu) cannot be found in '{local}'\n"
             "Runtime Platform missing or download incomplete"