Fix CI test changed file detect issue (#618)

Signed-off-by: chensuyue <suyue.chen@intel.com>

.github/workflows/_get-test-matrix.yml

@@ -51,7 +51,10 @@ jobs:
         run: |
           set -xe
           if [ "${{ github.event_name }}" == "pull_request" ] || [ "${{ github.event_name }}" == "pull_request_target" ]; then
-            base_commit=${{ github.event.pull_request.base.sha }}
+            LATEST_COMMIT_SHA=$(curl -H "Authorization: token ${{ secrets.GITHUB_TOKEN }}" \
+            "https://api.github.com/repos/opea-project/GenAIExamples/commits?sha=main" | jq -r '.[0].sha')
+            echo "Latest commit SHA is $LATEST_COMMIT_SHA"
+            base_commit=$LATEST_COMMIT_SHA
           else
             base_commit=$(git rev-parse HEAD~1) # push event
           fi