fix(agents): read selftest output via herestrings to avoid CI SIGPIPE - #729

On November 2, 2026, GitHub will restrict `pull_request_target` on public repositories by default. To continue allowing the event trigger, configure an Actions policy. Learn more: https://gh.io/securely-using-pull_request_target#default-policy-for-pull_request_target

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/create-github-app-token@v2, actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02, actions/upload-artifact@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

No file matched to [/home/runner/work/gpustack-operator/gpustack-operator/**/*requirements*.txt,/home/runner/work/gpustack-operator/gpustack-operator/**/*requirements*.in,/home/runner/work/gpustack-operator/gpustack-operator/**/*constraints*.txt,/home/runner/work/gpustack-operator/gpustack-operator/**/*constraints*.in,/home/runner/work/gpustack-operator/gpustack-operator/**/pyproject.toml,/home/runner/work/gpustack-operator/gpustack-operator/**/uv.lock,/home/runner/work/gpustack-operator/gpustack-operator/**/*.py.lock]. The cache will never get invalidated. Make sure you have checked out the target repository and configured the cache-dependency-glob input correctly.