FR: support uploading nightly artifact from a previous run

[neutrinoceros]

Inspired by https://github.com/matplotlib/matplotlib/blob/main/.github/workflows/nightlies.yml
I would like to propose and explore supporting a workflow where builds are separated from uploading. This would save CPU time by re-using previous build results.

The security implications seem acceptable and possibly non-existent to me; the currently supported workflow (re-build then upload) not being any more secure than this proposal, as far as I can tell.

Is there any appetite for it ?