Fix launching on Ubuntu Touch 20.04

APP_DIR is no longer an environment variable that can be used, but
current working directory does the same job so use it instead.

src/main.rs

@@ -102,9 +102,7 @@ fn init_gettext() {
     let domain = "webber.timsueberkrueb";
     textdomain(domain).expect("Failed to setup gettext domain");
 
-    let app_dir = env::var("APP_DIR").expect("Failed to read the APP_DIR environment variable");
-
-    let mut app_dir_path = PathBuf::from(app_dir);
+    let mut app_dir_path = env::current_dir().expect("Failed to get the app working directory");
     if !app_dir_path.is_absolute() {
         app_dir_path = PathBuf::from("/usr");
     }