workflows/cla.yml: use correct username for CLA check

[OddBloke]

Instead of using the username that triggered the action (which, in the
case of a committer merging master into a PR branch will be the
committer), always use the username of the submitter of the pull
request.

[OddBloke]

For reviewers:

• https://help.github.com/en/actions/reference/context-and-expression-syntax-for-github-actions is the relevant documentation for the github context
• https://developer.github.com/v3/activity/events/types/#pullrequestevent is the pull_request event documentation
• https://developer.github.com/v3/pulls/#response-1 is what the pull request object looks like

I have tested in my personal branch that this works for the trivial case (so we at least aren't regressing with this change): https://github.com/OddBloke/cloud-init/actions/runs/53241094/workflow

[blackboxsw]

Thanks for the extra doc links for context.