Make the updater work properly on SELinux (#518)

* Add DECKY_SELINUX env var

* if on selinux make binary executable with chcon

* No need to recursively change one file

backend/localplatform.py

@@ -40,4 +40,7 @@ def get_keep_systemd_service() -> bool:
 def get_log_level() -> int:
     return {"CRITICAL": 50, "ERROR": 40, "WARNING": 30, "INFO": 20, "DEBUG": 10}[
         os.getenv("LOG_LEVEL", "INFO")
-    ]
+    ]
+
+def get_selinux() -> bool:
+    return os.getenv("DECKY_SELINUX", "0") == "1"