Do not run Dependency Submission for external PRs

[maennchen]

rebar3-dependency-submission/.github/workflows/ci.yaml

Lines 80 to 83 in 259d3f9

	- name: Test local dependency submission
	run: |
	./_build/default/bin/rebar3_dependency_submission \
	--token "${{ secrets.GITHUB_TOKEN }}"

Will always fail with 403 for forks.