Fix mix-format for `.heex` files

[tomconroy]

Per the documentation for mix format:

  • --stdin-filename - path to the file being formatted on stdin. This is
    useful if you are using plugins to support custom filetypes such as .heex.
    Without passing this flag, it is assumed that the code being passed via
    stdin is valid Elixir code. Defaults to "stdin.exs".