Fix an import error in SK key support when fido2 package isn't present

This commit fixes an import error which occurred when the fido2 package
is not installed. Thanks go to GitHub user commonism for reporting this
issue!

asyncssh/sk.py

@@ -341,6 +341,7 @@ def sk_get_resident(application: str, user: Optional[str],
                       not ctypes.windll.shell32.IsUserAnAdmin()
 except (ImportError, OSError, AttributeError): # pragma: no cover
     sk_available = False
+    sk_use_webauthn = False
 
     def _sk_not_available(*args: object, **kwargs: object) -> NoReturn:
         """Report that security key support is unavailable"""