Feature request: allow to build international docs

[jeabakker]

In our project we also build translated version of the documentation, currently we can't test this with your action.

The command we use is: sphinx-intl build --locale-dir=docs/locale/
We can install sphinx-intl no problem (by adding it in the requirements.txt), but with the custom build-command we can't execute because some default arguments are added to any command which in this can makes sphinx-intl fail.

It would be nice if we somehow could build the international documentation.

Hope you can offer a solution