CI: prevent script injection

Treat github.event.pull_request.title as untrusted; use an intermediate
environment variable to prevent script injections.

Suggested-by: Stephen Paulger <Stephen.Paulger@arm.com>
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>

.github/workflows/main.yaml

@@ -27,6 +27,8 @@ jobs:
         # and only in the following cases:
         # - Push of a tag or to the main branch
         # - Pull request opened for the main branch of the main repository.
+        env:
+          PR_TITLE: ${{ github.event.pull_request.title }}
         if: ${{ github.repository == 'ARM-software/ebbr' }}
         run: |
           case "${{ github.event_name }}" in
@@ -43,7 +45,7 @@ jobs:
             if [ "${{ github.event.action }}" == opened ] &&
                [ "${{ github.event.pull_request.base.ref }}" == main ]; then
               msg="Pull request ${{ github.event.number }}"
-              msg="$msg (\"${{ github.event.pull_request.title }}\")"
+              msg="$msg (\"$PR_TITLE\")"
               msg="$msg ${{ github.event.action }}"
               url="${{ github.event.pull_request.html_url }}"
             fi ;;