chore(ci): bump NVHPC to 26.5 (#6117) * chore(ci): bump NVHPC to 26.5 Assisted-by: ClaudeCode:claude-opus-5 * chore(ci): download NVHPC deb with curl retries to dodge CDN 404s Assisted-by: ClaudeCode:claude-fable-5 --------- Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>