Look for $USER for the username before reading /etc/passwd

Let's take $USER into account if set before reading /etc/passwd
for the username. This gives a way out for environments where the
uid of the user does not have an entry in /etc/passwd.

mkosi/user.py

@@ -39,7 +39,7 @@ def is_running_user(cls) -> bool:
  @classmethod
  @functools.lru_cache(maxsize=1)
  def name(cls) -> str:
- return pwd.getpwuid(cls.uid).pw_name
+ return os.getenv("USER", pwd.getpwuid(cls.uid).pw_name)
 
  @classmethod
  @functools.lru_cache(maxsize=1)