Add missing environmental variable checks

Encryption keys, themes and general config dir (if used) were stored in
the default directories even if Moment-specific environmental variables
were present.

src/backend/matrix_client.py

@@ -165,7 +165,10 @@ class MatrixClient(nio.AsyncClient):
         device_id:  Optional[str] = None,
     ) -> None:
 
-        store = Path(backend.appdirs.user_data_dir) / "encryption"
+        store = Path(
+            os.environ.get("MOMENT_DATA_DIR") or backend.appdirs.user_data_dir,
+        ) / "encryption"
+
         store.mkdir(parents=True, exist_ok=True)
 
         proxy: Optional[str]