openapi action: enable pipefail

[brutasse]

This should prevent github actions from committing an empty spec to the repo in case of an HTTP failure. Example previous occurrence:

https://github.com/exoscale/python-exoscale/actions/runs/6809578290/job/18516145538

From the actions docs:

When shell: bash is specified, -o pipefail is also applied to enforce early exit from pipelines that generate a non-zero exit status.