Potential speed improvement: parallelise 🔒 Read Signing Key Id & 🎊 Test & Version

[rtyley]

🔒 Read Signing Key Id and 🎊 Test & Version could be parallelised before 🔒 Push Release Commit:

🎊 Test & Version doesn't really need to know the GPG key id or email address, though it currently does make some commits, and that needs some email address.

A quick option to fix the issue might be to use a stub email address in 🎊 Test & Version, though we would need to make sure that 🔒 Push Release Commit ends up using the correct committer address if it's cherry-picking those commits from 🎊 Test & Version.

A better option may be to switch from communicating between jobs using bare-git-repo, and instead just have 🎊 Test & Version output just the crucial version number. The subsequent jobs do need to know which file to modify (ie version.sbt), how to construct the version tag (ie, add a v prefix) and also the snapshot version number of the next version.