fix `set +euo pipefail` in travis_retry and docker_retry

[SimeonEhrig]

If the functions was called the changed bash behavior was still working after the function call.

~Maybe fixes: #2357~ Does not solve #2357, see test PR: https://github.com/alpaka-group/alpaka/pull/2356

The PR fixes also two bugs in the CI scripts:

• use unbound variable LD_LIBRARY_PATH in install_cuda.sh
• use g++-12 in custom jobs, which is not available on Ubuntu 20.04