Release workflow installs TestCoverage rather than loading

This produces an artifact for the latest version published to IPM, rather than the version of the code as of the tag, which is wrong. (I'd prefer the normal workflow to be releasing on GitHub and *then* releasing in Open Exchange.)

While we're at it, let's see if the release we're about to create already exists and add the artifact to it if it does (rather than overwriting the release description as it currently does)