nixos-rebuild-ng: use Final in constants.py

from typing import Final

# Build-time flags

# Use strings to avoid breaking standalone (e.g.: `python -m nixos_rebuild`)

# usage

EXECUTABLE = "@executable@"

EXECUTABLE: Final[str] = "@executable@"

# Use either `== "true"` if the default (e.g.: `python -m nixos_rebuild`) is

# `False` or `!= "false"` if the default is `True`

WITH_NIX_2_18 = "@withNix218@" != "false"  # type: ignore

WITH_REEXEC = "@withReexec@" == "true"  # type: ignore

WITH_SHELL_FILES = "@withShellFiles@" == "true"  # type: ignore

WITH_NIX_2_18: Final[bool] = "@withNix218@" != "false"

WITH_REEXEC: Final[bool] = "@withReexec@" == "true"

WITH_SHELL_FILES: Final[bool] = "@withShellFiles@" == "true"