amend! machine.linux: Fix a mypy error

machine.linux: Fix a mypy error

Mypy was complaining that

    tbot/machine/linux/build.py:65:
    error: Argument 2 to "exec0" of "LinuxShell" has incompatible type
    "Path[H@__init__]"; expected "Union[str, Special[H@enable],
    Path[H@enable]]"

Tweak the code so mypy correctly understands the relationship between
types.

tbot/machine/linux/build.py

@@ -21,14 +21,11 @@
 from . import linux_shell, path
 
 
-H = typing.TypeVar("H", bound="Builder")
-
-
-class Toolchain(abc.ABC, typing.Generic[H]):
+class Toolchain(abc.ABC):
     """Generic toolchain type."""
 
     @abc.abstractmethod
-    def enable(self, host: H) -> None:
+    def enable(self, host: "Builder") -> None:
         """Enable this toolchain on the given ``host``."""
         pass
 
@@ -52,15 +49,15 @@ def toolchains(self):
             }
     """
 
-    def __init__(self, path: path.Path[H]) -> None:
+    def __init__(self, path: "path.Path[Builder]") -> None:
         """
         Create a new EnvScriptToolchain.
 
         :param linux.Path path: Path to the env script
         """
         self.env_script = path
 
-    def enable(self, host: H) -> None:
+    def enable(self, host: "Builder") -> None:
         host.exec0("unset", "LD_LIBRARY_PATH")
         host.exec0("source", self.env_script)
 
@@ -85,7 +82,7 @@ def __init__(self, arch: str, prefix: str) -> None:
         self.arch = arch
         self.prefix = prefix
 
-    def enable(self, host: H) -> None:
+    def enable(self, host: "Builder") -> None:
         host.env("ARCH", self.arch)
         host.env("CROSS_COMPILE", self.prefix)