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

@@ -30,9 +30,6 @@ def enable(self, host: "Builder") -> None:
         pass
 
 
-H = typing.TypeVar("H", bound="Builder")
-
-
 class EnvScriptToolchain(Toolchain):
     """
     Toolchain that is initialized using an env script (e.g. yocto toolchain).
@@ -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)
 

tbot/tc/selftest/tc/build.py

@@ -30,7 +30,7 @@ def toolchains(self) -> typing.Dict[str, linux.build.Toolchain]:
         """Return toolchains available on this host."""
         return {
             "selftest-toolchain": linux.build.EnvScriptToolchain(
-                self.workdir / ".selftest-toolchain.sh"
+                self.workdir / ".selftest-toolchain.sh"  # type: ignore
             )
         }