ci: maintainer_check: Check out PR MAINTAINERS file

The commit 31ef45e2 converted the
maintainer check workflow from `pull_request` to `pull_request_target` --
this caused the "checkout" action to check out the base branch instead of
the pull request branch and effectively made the workflow compare the
mainline MAINTAINERS.yml against itself.

This commit reworks the workflow such that it checks out the pull request
MAINTAINERS.yml separately and compare it against the mainline
MAINTAINERS.yml checked out by the "checkout" action.

Note that the `requirements-actions.txt` and `check_maintainer_changes.py`
files used in this workflow are checked out from the base branch (i.e. does
not contain any modifications) and therefore do not pose a security risk.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>