portable: don't override portable user preference if not PyInstaller

The previous code would set `portable` to `False` when not ran using
PyInstaller, overriding user preference. This meant that AppImage could
not be used portable.

We now only do the portable auto detection when the user hasn't passed
the `--portable` flag already.

electron-cash

@@ -444,15 +444,14 @@ def process_config_options(args):
         config_options["auto_connect"] = False
     config_options["cwd"] = os.getcwd()
 
-    # fixme: this can probably be achieved with a runtime hook (pyinstaller)
-    try:
-        tmp_folder = os.path.join(
-                sys._MEIPASS,  # pylint: disable=W0212
-                "is_portable"
-        )
-        config_options["portable"] = is_bundle and os.path.exists(tmp_folder)
-    except AttributeError:
-        config_options["portable"] = False
+    if not config_options.get("portable"):
+        # auto detect whether we are started from the portable bundle
+        # we only do this when the user has not already specified portable mode
+        # fixme: this can probably be achieved with a runtime hook (pyinstaller)
+        meipass = getattr(sys, "_MEIPASS", None)
+        if meipass:
+            tmp_folder = os.path.join(meipass, "is_portable")
+            config_options["portable"] = is_bundle and os.path.exists(tmp_folder)
 
     if config_options.get("portable"):
         config_options["electron_cash_path"] = os.path.join(