Prevent running publishing workflow on forks

[michaelwheeler]

The test-publish job will fail with running from a fork.

django-lti/.github/workflows/packaging.yml

Lines 26 to 43 in 72703a6

	test-publish:
	needs: build
	runs-on: ubuntu-latest
	steps:
	- uses: actions/setup-python@v2
	with:
	python-version: "3.9"
	- uses: actions/download-artifact@v3
	with:
	name: package-build
	path: dist/
	- uses: pypa/[email protected]
	with:
	user: __token__
	password: ${{ secrets.TEST_PYPI_API_TOKEN }}
	repository_url: https://test.pypi.org/legacy/
	skip_existing: true
	verbose: true