OneDev mounts a directory to be used as cache (or create symbol link when shell executor is used instead of docker executor), and does not have any special "save cache" or "upload cache" action. Even if build is timed out or failed, the file written to cache will still be available for subsequent builds.

I tested with below build spec:

version: 13
jobs:
- name: CI
  steps:
  - !CommandStep
    name: build
    image: alpine
    commands:
    - cd npm
    - touch somefile
    - sleep 60
    useTTY: false
    condition: ALL_PREVIOUS_STEPS_WERE_SUCCESSFUL
  retryCondition: never
  maxRetries: 3
  retryDelay: 30
  cpuRequirement: 500
  memoryRequirement: 256
  caches:
  - key: npm
    path: npm
  timeout: 10

After build finishes, check directory <OneDev install dir>/cache/npm/<some uuid> and you will find touched file somefile there.