actions: add maintainers as PR reviewers for their packages (#12269)

Use `spack pkg changed` and `spack maintainers` to figure out which
packages changed and who their maintainers are in a PR.

Add any maintainers to the PR as reviewers.

.github/actions/add-maintainers-as-reviewers/Dockerfile

@@ -0,0 +1,6 @@
+FROM python:3.7-alpine
+
+RUN pip install pygithub
+
+ADD entrypoint.py /entrypoint.py
+ENTRYPOINT ["/entrypoint.py"]