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(...)
diff --git a/.github/workflows/vtest.yml b/.github/workflows/vtest.yml
index c88d4aa..4d6fdfb 100644
--- a/.github/workflows/vtest.yml
+++ b/.github/workflows/vtest.yml
@@ -23,7 +23,7 @@
       - uses: actions/checkout@v2
       - name: Generate Build Matrix
         id: set-matrix
-        run: python3 .github/matrix.py
+        run: python3 .github/matrix.py "${{ github.event_name }}"
 
   # The Test job actually runs the tests.
   Test: