--use-hardened-images doesn't work on Drupal6/Drupal7 because drush link fails

[rfay]

Describe the bug

The "--use-hardened-images" production feature doesn't work with Drupal6/7 because the attempt to link drush to drush8 doesn't work.

To Reproduce

Using ddev v1.17.0-alpha2:

• ddev config --use-hardened-images
• In a Drupal7 project, ddev start

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Version and configuration information (please complete the following information):

• Host computer OS and Version: [e.g. Windows 10, macOS Catalina]
• Docker Desktop version if on macOS or Windows (from "About Docker Desktop")
• ddev version information (use ddev version)
• config.yaml contents for the misbehaving project
• Do you have any custom configuration (nginx, php, mysql) in the .ddev folder? If so, have you tried without them?

Additional context
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