✨ Improve ergonomics for dagger shell scripts

[jedevc]

What are you trying to do?

Currently, it's a bit fiddly to execute a dagger shell script - we should probably have a better way of doing this.

How are you currently working around this?

The best to do this today is to write the desired script to a file (like /tmp/dagger-shell-script), and then run it using verb: shell and args: /tmp/dagger-shell-script.

This is a bit fiddly, and since we've now released shell, it should be easier to call it from this action.