Buildcache commands cleanup, again... (#39203)

* Inform mypy that tty.die is noreturn

* avoid temporary allocation in env

* update spack buildcache save-specfile

* fix spack buildcache check/download/get-buildcache-name

- ensure that required args and mutually exclusive ones are marked as
  such in argparse for better error messages
- deprecate --spec-file everywhere
- use disambiguate for better error messages

lib/spack/llnl/util/tty/__init__.py

@@ -12,6 +12,7 @@
 import traceback
 from datetime import datetime
 from sys import platform as _platform
+from typing import NoReturn
 
 if _platform != "win32":
     import fcntl
@@ -244,7 +245,7 @@ def warn(message, *args, **kwargs):
     info("Warning: " + str(message), *args, **kwargs)
 
 
-def die(message, *args, **kwargs):
+def die(message, *args, **kwargs) -> NoReturn:
     kwargs.setdefault("countback", 4)
     error(message, *args, **kwargs)
     sys.exit(1)