fix(submit): allow submit to be called by nested composite actions

[rene-oromtz]

Description

The current github_ref that is used so the testflinger-setup has the same reference as the submit action and it work in almost all but one edge case.

Whenever calling the submit action from another composite action in "Repo B" which is then called from a third repo called "Repo C" and using a specified hash or tag, it will not honor the revision from which the submit action is called e.g.
repoC: uses actionB@my-branch
repoB: actionB uses submit@submit/v1.4.4

The github_ref will then be set to my-branch which does not exist in this repository causing a failure.
To enable developers to test local changes on composite action, this PR changes to github.action_path to create a local copy of the testflinger-setup. This uses the same reference as the submit action without needing a github_ref

Resolved issues

NA

Documentation

Web service API changes

Tests

Tested on this run for backward compatibility calling this submit action from another workflow: https://github.com/canonical/tf-submit-action-test/actions/runs/24295950198/job/70940894955

Dev has also confirmed this approach works on this MM thread: https://chat.canonical.com/canonical/pl/w3g3pi4d5insfb5ndny7srzema

[ajzobro]

Looks good to me, not sure if my understanding of the comment is right though.

[rene-oromtz]

@ajzobro thanks for the review! I just pushed your suggestions regarding the comments

[ajzobro]

LGTM