[CI] enable fail-fast for merge-group runs

[mitchdz]

Summary

Enable fail-fast: true for matrix jobs when the workflow is triggered by a merge group (merge queue). For normal PR/push runs, matrix jobs keep fail-fast: false so the full matrix still runs for debugging.

Motivation

When a merge-group run has a failing job, we want the run to fail quickly so the merge group is marked failed and the queue can move on to the next group. With fail-fast enabled only for merge-group:

• The first failing matrix combination cancels the rest of that job’s matrix (saves time and runners).
• The workflow run fails as soon as that job fails, so the merge group check fails and the next groups can start sooner.
• Regular PR and push runs are unchanged and still run the full matrix for debugging.

Changes

• Set fail-fast: ${{ github.event_name == 'merge_group' }} on all matrix jobs: devdeps, wheeldeps, source_build, build_and_test, gen_code_coverage, docker_image, python_wheels, and binaries.

[github-actions]

CUDA Quantum Docs Bot: A preview of the documentation can be found here.

[github-actions]

CUDA Quantum Docs Bot: A preview of the documentation can be found here.

[1tnguyen]

👍