.github/workflows/trigger-scylla-ci: fix heredoc injection in trigger-scylla-ci workflow

Move all ${{ }} expression interpolations into env: blocks so they are
passed as environment variables instead of being expanded directly into
shell scripts. This prevents an attacker from escaping the heredoc in
the Validate Comment Trigger step and executing arbitrary commands on
the runner.

The Verify Org Membership step is hardened in the same way for
defense-in-depth.

Refs: GHSA-9pmq-v59g-8fxp
Fixes: SCYLLADB-954

Closes scylladb/scylladb#28935

(cherry picked from commit 977bdd6260)

Closes scylladb/scylladb#28946

.github/workflows/trigger-scylla-ci.yaml

@@ -15,13 +15,19 @@ jobs:
       - name: Verify Org Membership
         id: verify_author
         env:
-          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          EVENT_NAME: ${{ github.event_name }}
+          PR_AUTHOR: ${{ github.event.pull_request.user.login }}
+          PR_ASSOCIATION: ${{ github.event.pull_request.author_association }}
+          COMMENT_AUTHOR: ${{ github.event.comment.user.login }}
+          COMMENT_ASSOCIATION: ${{ github.event.comment.author_association }}
         shell: bash
         run: |
-          if [[ "${{ github.event_name }}" == "pull_request_target" ]]; then
-            AUTHOR="${{ github.event.pull_request.user.login }}"
+          if [[ "$EVENT_NAME" == "pull_request_target" ]]; then
+            AUTHOR="$PR_AUTHOR"
+            ASSOCIATION="$PR_ASSOCIATION"
           else
-            AUTHOR="${{ github.event.comment.user.login }}"
+            AUTHOR="$COMMENT_AUTHOR"
+            ASSOCIATION="$COMMENT_ASSOCIATION"
           fi
           ORG="scylladb"
           if gh api "/orgs/${ORG}/members/${AUTHOR}" --silent 2>/dev/null; then
@@ -34,13 +40,11 @@ jobs:
       - name: Validate Comment Trigger
         if: github.event_name == 'issue_comment'
         id: verify_comment
+        env:
+          COMMENT_BODY: ${{ github.event.comment.body }}
         shell: bash
         run: |
-          BODY=$(cat << 'EOF'
-          ${{ github.event.comment.body }}
-          EOF
-          )
-          CLEAN_BODY=$(echo "$BODY" | grep -v '^[[:space:]]*>')
+          CLEAN_BODY=$(echo "$COMMENT_BODY" | grep -v '^[[:space:]]*>')
 
           if echo "$CLEAN_BODY" | grep -qi '@scylladbbot' && echo "$CLEAN_BODY" | grep -qi 'trigger-ci'; then
             echo "trigger=true" >> $GITHUB_OUTPUT