Add support for using an environment for the upload step

[Cadair]

It would be cool if you could specify what environment to use just for the upload step rather than the current situation where you have to specify it for the whole publish workflow.

This would allow someone to approve the upload after all the builds complete.

[ConorMacBride]

Unless the job environment input has a default value, we would have to duplicate the upload job: one job if an environment input is passed to the workflow and another if it isn't. Is that adding too much complexity? Especially with the Anaconda.org upload step, this would be a lot of duplicated code.

Also, the upload step for publish_pure_python.yml does not have a separate job for upload. Move to a separate job or apply environment to whole workflow?

[Cadair]

Does referencing an environment which doesn't exist work?!

Could we use a composite action to reduce the duplication?

[Cadair]

Looking into this a bit more, it seems you can use environment: and with: in the same block, so we will have to add explicit support.