Use f-string with formatting to convert octal permissions to string

Previous method produced incorrect results if user permissions were
set to zero.

src/bubblejail/bwrap_config.py

@@ -29,7 +29,7 @@ def __init__(self, permissions: Optional[int] = None):
     def to_args(self) -> Generator[str, None, None]:
         if self.permissions is not None:
             yield "--perms"
-            yield oct(self.permissions).lstrip("0o")
+            yield f"{self.permissions:04o}"
 
         yield from super().to_args()