ci: fix docpublish workflow

It seems that it's possible to inject envs with current code.
Fixing this issue.

Signed-off-by: Kari Hamalainen <[email protected]>

Author: karhama

.github/workflows/docpublish.yml

@@ -31,7 +31,10 @@ jobs:
       - name: Obtain PR number
         working-directory: docs
         run: |
-          echo "PR=$(cat pr.txt)" >> "$GITHUB_ENV"
+          if [ -f pr.txt ]; then
+            PR=$(head -n 1 pr.txt | tr -cd '0-9')
+          fi
+          printf 'PR=%s\n' "$PR" >> "$GITHUB_ENV"
 
       - name: Upload to Azure storage
         working-directory: docs