Revert to pull_request_target workflow trigger

This seems to be required for image build & scanning to work (e.g. [here](https://github.com/stackhpc/azimuth-llm/actions/runs/11655504230/job/32450298649#step:8:2319). We explicity check out the correct ref in chart_validation to match other workflow steps and ensure we're testing the correct version of the chart.

.github/workflows/test-pr.yml

@@ -2,7 +2,7 @@
 name: Test pull request
 
 on:
-  pull_request:
+  pull_request_target:
     types:
       - opened
       - synchronize
@@ -41,6 +41,8 @@ jobs:
     steps:
       - name: Checkout the repo
         uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
 
       - name: Install Helm
         uses: azure/setup-helm@v4