PR validation: write the pullrequest body as a separate file and an end marker (#37030)

* PR validation: write the pullrequest body as a separate file with a echo marker

This avoids having quotes or other text considered special. Tried to
name the marker in a way that is reasonably unique and unlikely to be
part of real PR summaries. WE can change it more if we really need to.

* Minor change to kick CI

Author: andy31415

.github/workflows/pr-validation.yaml

@@ -8,18 +8,25 @@ jobs:
   check_testing_header:
     runs-on: ubuntu-latest
     steps:
-
       - name: Check for `### Testing` section in PR
         id: check-testing
         continue-on-error: true
         run: |
-          python -c 'import sys; pr_summary = """${{ github.event.pull_request.body }}"""; sys.exit(0 if "### Testing" in pr_summary else 1)'
+          cat >/tmp/pr-summary.txt << "EndMarkerForPrSummary"
+          ${{ github.event.pull_request.body }}
+          EndMarkerForPrSummary
+
+          python -c 'import sys; pr_summary = open("/tmp/pr-summary.txt", "rt").read(); sys.exit(0 if "### Testing" in pr_summary else 1)'
 
       - name: Check for PR starting instructions
         id: check-instructions
         continue-on-error: true
         run: |
-          python -c 'import sys; pr_summary = """${{ github.event.pull_request.body }}"""; sys.exit(1 if "Make sure you delete these instructions" in pr_summary else 0)'
+          cat >/tmp/pr-summary.txt << "EndMarkerForPrSummary"
+          ${{ github.event.pull_request.body }}
+          EndMarkerForPrSummary
+
+          python -c 'import sys; pr_summary = open("/tmp/pr-summary.txt", "rt").read(); sys.exit(1 if "Make sure you delete these instructions" in pr_summary else 0)'
 
       # NOTE: comments disabled for now as separate permissions are required
       #       failing CI step may be sufficient to start (although it contains less information about why it failed)