Allow action inputs to be read from `pyproject.toml`

[ConorMacBride]

Picking up on OpenAstronomy/github-actions-workflows#127 (comment) I wonder if we should make the sdist action be able to read its config from pyproject.toml then we wouldn't need these vars at all?

Originally posted by @Cadair in OpenAstronomy/github-actions-workflows#126 (comment)