Fix minor bug involving locally cached tasks and no schedule

[cicdw]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

Note that your PR will not be reviewed unless all three boxes are checked.

What does this PR change?

Flows which:

• run with run_on_schedule=False
• have a cached task
  would error out with:

AttributeError: 'Context' object has no attribute 'caches'

because of the place that we set this key. This PR moves that key-setting to a more universal location and adds a test.

Why is this PR important?

For local development and testing!

[codecov]

Codecov Report

Merging #1599 into master will not change coverage.
The diff coverage is 100%.