fix: add shell: bash to PDF release version step (container uses sh)

Signed-off-by: skjnldsv <skjnldsv@protonmail.com>

Author: skjnldsv

.github/workflows/sphinxbuild.yml

@@ -271,6 +271,7 @@ jobs:
 
       - name: Compute PDF release version
         id: pdf_version
+        shell: bash
         run: |
           # For PRs use the target branch; for pushes use the current branch.
           # This handles both stable34 direct pushes and backport/*/stable34 PRs.