Support automated build for latest ghidra version

[JayFoxRox]

https://github.com/er28-0652/setup-ghidra#version supports "latest" as version.
(Edit: I tried using it, but it didn't work, see er28-0652/setup-ghidra#14)

Support for this should be added to the Actions workflow, so the nightly build is always up-to-date (avoiding some of #60, #64, #65).
This definitely makes sense for the nightly workflow, maybe not so much for publish workflow (as a sudden new release could break a final build).
For the testing workflow it depends (as frequent breakage would be more likely, but it might not be intended to have a "build broken" badge in the README).