actions: add trivial matrix example (#912)

Reviewed-on: https://code.forgejo.org/forgejo/end-to-end/pulls/912
Reviewed-by: Gusted <gusted@noreply.code.forgejo.org>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

actions/example-matrix/.forgejo/workflows/test.yml

@@ -0,0 +1,12 @@
+on: [push]
+jobs:
+  test:
+    runs-on: docker
+    name: name-${{ matrix.version }}
+    strategy:
+      matrix:
+        version: [1.18, 1.19]
+    steps:
+      - run: |
+         set -x
+         test "${{ matrix.version }}"