Set / bump __version__ for all the runner packages during the release

[Kami]

This pull request updates release workflow to set __version__ for all the runner packages during the release process.

This change is part of StackStorm/st2#4239.

Right now this change is not backward compatible which means we need to decide how to handle backward compatibility for v2.8:

1. Leave this code as is and cherry pick Update runner packages to read version from runner package local __init__.py file st2#4239 into v2.8 branch (should be fine)
2. Update the workflow so setting runner version is not fatal if runner package __init__.py file doesn't contain __version__ string.

That's fine for v2.8, but I don't like it going forward (it could mask actual issues) - I prefer failing loudly where possible.

[arm4b]

Is think too that, cherry picking (1) sounds good enough & easiest, so we don't overcomplicate it.

[Kami]

Alright, I will make that change.

If we ever need to release patch for version < v2.8 we can make corresponding changes to this workflow.