[ci] - Don't cache habitat-lab

[aclegg3]

Motivation and Context

For some reason habitat-lab caching was broken resulting in a stale commit failing tests. For now we'll not cache while we figure out the issue.

Note: also adds hab_stretch to ci_test_assets as required by new lab tests.

How Has This Been Tested

CI is passing again.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.