CI: Pass the github.event_name to matrix.py

CI: Pass the github.event_name to matrix.py

This is a preparation to later run some matrix entries on schedule only.

Within the matrix.py script it can now be detected whether the workflow is
running on schedule by using:

    if build_type == "schedule":
        matrix.append(...)

(cherry picked from commit 8d173e17c0187b551a7f14e62e0d8d16c773fb55)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>