Add the ability to specify a custom working directory for tox and custom build directory for publish

[astrofrog]

This requires OpenAstronomy/build-python-dist#27 so won't work until that's merged.

Note that the option is called working-directory for tox and build-directory for publish, does that make sense or should we change that?

[ConorMacBride]

I think working-directory would make sense for both workflows, as the default build srcdir is the working directory.

[astrofrog]

@ConorMacBride - I think this is ready for review!

[ConorMacBride]

This all looks good. We should just specify that working-directory cannot be overwritten per tox environment here beside submodules https://github-actions-workflows--358.org.readthedocs.build/en/358/tox.html#inputs

[astrofrog]

@ConorMacBride - I think I've addressed your comment