Detect the invoking user when running with doas

User detection only checked SUDO_USER, so with doas the user's
config file was not found unless -u was given. Fall back to
DOAS_USER, as already done for notifications.

simple_backup/simple_backup.py

@@ -644,14 +644,15 @@ class Backup:
             return 0
 
 
+def _detect_user() -> Optional[str]:
+    if os.geteuid() == 0:
+        return os.getenv('SUDO_USER') or os.getenv('DOAS_USER')
+
+    return os.getenv('USER')
+
+
 def _parse_arguments() -> argparse.Namespace:
-    euid = os.geteuid()
-
-    if euid == 0:
-        user = os.getenv('SUDO_USER')
-    else:
-        user = os.getenv('USER')
-
+    user = _detect_user()
     homedir = os.path.expanduser(f'~{user}')
 
     parser = argparse.ArgumentParser(prog='simple_backup',
@@ -874,7 +875,7 @@ def simple_backup() -> int:
         euid = os.geteuid()
 
         if euid == 0:
-            user = os.getenv('SUDO_USER')
+            user = _detect_user()
 
             if user is not None:
                 homedir = os.path.expanduser(f'~{user}')