Make posix and msys2 shells consistent in CI

Make posix and msys2 shells consistent in CI

In CI, it is typical for the process to halt immediately when an error
is encountered. However, with our `shell: bash {0}` configuration,
the process continues despite errors for posix shells.
This commit updates the behavior of posix and msys2 shells to ensure
consistency in terms of pipeline exit codes and stop conditions.
We adopt the most appropriate default behavior as recommended
by the GitHub documentation.

Update the code that searches for the bench value in the git log:
- to be compatible with the new shell settings
- to retry the value from the first line that contains
  only the template and spaces/tabs/newlines

see also

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idstepsshell
https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#exit-codes-and-error-action-preference
https://github.com/msys2/setup-msys2/blob/main/main.js

closes https://github.com/official-stockfish/Stockfish/pull/4653

No functional change