Make alerting survive -e and always write the run summary

[danielhanchen]

Same fix as unslothai/llama.cpp#62, where I found it. The action is a verbatim copy in both repos, and has_issues=false here too.

Two bugs. GitHub invokes shell: bash with -e, which set -uo pipefail does not undo, so the step died on the first non-zero command with no output despite being written to tolerate failures. And every alert ends in gh issue create, which cannot succeed while Issues are disabled on the fork, so no alert this pipeline raises has ever been readable.

The details now go to $GITHUB_STEP_SUMMARY before the API is touched, which needs no token or repository setting, and a failed creation reports the actual cause rather than a generic message.