neovim: unify staging code for test and deploy

[pokey]

We currently have one script to create a staging directory for testing in CI, and a completely separate script to create the staging directory for deploy. We should try to unify these as much as possible so that we can have more confidence that we're testing the setup that will actually ship

We could potentially use one script for both, and then add two new scripts that run after the first script:

• one script runs only in test and just adds the test files
• one script runs only in deploy and does an rsync to the staging repo directory

Possibly related to #2589