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/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
             )
         }