umu_consts: fix runtime path for flatpak

- XDG_DATA_HOME would only be valid for the umu flatpak or if a client is calling our flatpak app. Since Heroic and Lutris are including umu in their runtime, with the current logic, the runtime will be stored in their private home directory which is not what we want

umu/umu_consts.py

@@ -38,7 +38,11 @@ class Color(Enum):
 FLATPAK_ID = os.environ.get("FLATPAK_ID") or ""
 
 FLATPAK_PATH: Path | None = (
-    Path(os.environ["XDG_DATA_HOME"], "umu") if FLATPAK_ID else None
+    Path.home().joinpath(
+        ".var", "app", "org.openwinecomponents.umu.umu-launcher", "data", "umu"
+    )
+    if FLATPAK_ID
+    else None
 )
 
 UMU_LOCAL: Path = FLATPAK_PATH or Path.home().joinpath(