app: when interpreter not found in interpreter DB, leave shebang as-is.

In particular, if the shebang is `#!/usr/bin/env lang` and we have not
registered a specific interpreter for `lang`, the system should leave
the env tool search the right interpreter for us. We only bypass env
when we set our own explicit interpreter.

Apply this to palette-to-gradient.py plug-in.

plug-ins/python/palette-to-gradient.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by