github-ci: error on any shell errors

github-ci: error on any shell errors

Enable `errexit` and `nounset` [POSIX shell options][1]
in `.github/workflows/entrypoint.sh` so that the script fails
if any command within the script fails.

[1]: https://pubs.opengroup.org/onlinepubs/9699919799//utilities/V3_chap02.html#set

Reported-by: Marius Dinu <m95d+git@psihoexpert.ro>
Fixes: https://github.com/openwrt/packages/issues/19953
Signed-off-by: Alois Klink <alois@aloisklink.com>
(cherry picked from commit 18d3c529fa4ab8dafcf6c147cf3cb55792d8ca10)