[fix] AppImage advanced settings location (#3866)

Reported on the forum by roger64: https://www.mobileread.com/forums/showthread.php?t=296184

frontend/apps/filemanager/filemanagersetdefaults.lua

@@ -14,10 +14,14 @@ local util = require("ffi/util")
 local _ = require("gettext")
 local Screen = require("device").screen
 
+local is_appimage = os.getenv("APPIMAGE")
+
 local function getDefaultsPath()
     local defaults_path = DataStorage:getDataDir() .. "/defaults.lua"
     if isAndroid then
         defaults_path = android.dir .. "/defaults.lua"
+    elseif is_appimage then
+        defaults_path = "defaults.lua"
     end
     return defaults_path
 end