Deploy PR to staging only if CI checks passed

Although there are various checks enabled in KernelCI repositories (e.g. https://github.com/kernelci/kernelci-pipeline/pull/643) it is not verified if they all passed before [adding a PR to the staging deployment](https://github.com/kernelci/kernelci-deploy/blob/8674f1b1bc77a8bc5349319e9fbd008060a80e1f/pending.py#L77).

PRs should be included in staging on condition that all CI checks passed.

Start in pending.py at the linked staging-deployment logic around line 77, then review the checks referenced by kernelci-pipeline pull request 643. Trace how a pull request is selected for staging and define the gate based on all CI checks passing. Done means a PR with any failed or incomplete check is excluded from staging, while one with all checks passed can be included.

Written by the indexing model from the issue text.