Optimize preview build to avoid checkout on pull_request* events (#633)

Author: reakaleek

.github/workflows/preview-build.yml

@@ -46,17 +46,24 @@ jobs:
     steps:
 
       - name: Checkout
+        if:  contains(fromJSON('["push", "merge_group"]'), github.event_name)
         uses: actions/checkout@v4
         with:
           ref: ${{ github.event.pull_request.head.sha || github.ref }}
 
       - name: Get changed files
-        if: startsWith(github.event_name, 'pull_request') || github.event_name == 'merge_group'
+        if: contains(fromJSON('["merge_group", "pull_request", "pull_request_target"]'), github.event_name)
         id: check-files
         uses: tj-actions/changed-files@d6e91a2266cdb9d62096cebf1e8546899c6aa18f # v45.0.6
         with:
           files: ${{ inputs.path-pattern != '' && inputs.path-pattern || '**' }}
-
+          
+      - name: Checkout
+        if: startsWith(github.event_name, 'pull_request') && steps.check-files.outputs.any_modified == 'true'
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha || github.ref }}
+          
       - name: Free Disk Space
         if: github.repository == 'elastic/asciidocalypse'
         uses: jlumbroso/free-disk-space@54081f138730dfa15788a46383842cd2f914a1be # v1.3.1