Make ``JoinedStr`` nodes ``Uninferable`` when the ``FormattedValue`` is ``Uninferable``. (#2944)

Ensure ``ast.JoinedStr`` nodes are ``Uninferable`` when the ``ast.FormattedValue`` is
``Uninferable``. This prevents ``unexpected-keyword-arg`` messages in Pylint
where the ``Uninferable`` string appeared in function arguments that were
constructed dynamically.

Closes pylint-dev/pylint#10822