ci: update PR trigger for mshv workflow

Pull requests from forked repos cannot access GitHub secrets which
results in failure of MSHV CI. Switching to pull_request_target
resolves this. It allows the workflow to run with access to repo secrets
and ensures that code from the base branch is used instead of forked
code, preventing potential security risks.

Signed-off-by: AASTHA RAWAT <aastharawat@microsoft.com>

.github/workflows/mshv-integration.yaml

@@ -1,9 +1,5 @@
 name: MSHV Integration Tests
-on:
-  push:
-    branches:
-      - test_mshv_ci
-  pull_request:
+on: [pull_request_target, merge_group]
 
 jobs:
   infra-setup:
@@ -27,21 +23,14 @@ jobs:
     needs: infra-setup
     if: ${{ always() && needs.infra-setup.result == 'success' }}
     runs-on: mshv
+    continue-on-error: true
     steps:
-      - name: Determine branch to build
-        run: |
-          echo "Determining branch to build and test..."
-          if [[ "${{ github.event_name }}" == "pull_request" ]]; then
-            echo "BRANCH=${{ github.event.pull_request.head.ref }}" >> $GITHUB_ENV
-          else
-            echo "BRANCH=${{ inputs.branch }}" >> $GITHUB_ENV
-          fi
-
       - name: Run integration tests
         env:
-          BRANCH_NAME: ${{ env.BRANCH }}
           KEY: azure_key_${{ github.run_id }}
           PR_NUMBER: ${{ github.event.pull_request.number }}
+          REPO_URL: https://github.com/cloud-hypervisor/cloud-hypervisor.git
+          REPO_DIR: cloud-hypervisor
           PRIVATE_IP: ${{ needs.infra-setup.outputs.PRIVATE_IP }}
           RG: MSHV-${{ github.run_id }}
           USERNAME: ${{ secrets.MSHV_USERNAME }}
@@ -52,11 +41,18 @@ jobs:
             set -e
             echo "Logged in successfully."
             export PATH="\$HOME/.cargo/bin:\$PATH"
-            echo "${BRANCH_NAME}"
-            git clone --depth 1 --single-branch --branch "$BRANCH_NAME" https://github.com/cloud-hypervisor/cloud-hypervisor.git
-            cd cloud-hypervisor
 
-             echo "Loading VDPA kernel modules..."
+            if [[ "${{ github.event_name }}" == "pull_request_target" ]]; then
+              git clone --depth 1 "$REPO_URL" "$REPO_DIR"
+              cd "$REPO_DIR"
+              git fetch origin pull/${{ github.event.pull_request.number }}/merge
+              git checkout FETCH_HEAD
+            else
+              git clone --depth 1 --single-branch --branch "${{ github.ref_name }}" "$REPO_URL" "$REPO_DIR"
+              cd "$REPO_DIR"
+            fi
+
+            echo "Loading VDPA kernel modules..."
             sudo modprobe vdpa
             sudo modprobe vhost_vdpa
             sudo modprobe vdpa_sim