Fix inverse logic in hierarchy based link resolver environment opt-out

[d-ronnqvist]

Bug/issue #, if applicable:

Summary

This fixes an issue where the opt-logic for the new link resolver was inverted.

Dependencies

None.

Testing

• Run swift run docc convert /path/to/SomeBundle.docc with DOCC_USE_HIERARCHY_BASED_LINK_RESOLVER=YES.
  DocC should use the hierarchy based resolver.
• Run swift run docc convert /path/to/SomeBundle.docc without DOCC_USE_HIERARCHY_BASED_LINK_RESOLVER set.
  DocC should use the hierarchy based resolver.
• Run swift run docc convert /path/to/SomeBundle.docc with DOCC_USE_HIERARCHY_BASED_LINK_RESOLVER=NO.
  DocC shouldn't use the hierarchy based resolver.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [ ] Added tests
• Ran the ./bin/test script and it succeeded
• [ ] Updated documentation if necessary

[d-ronnqvist]

@swift-ci please test

[QuietMisdreavus]

What do you think about having the environment variable override the user default here? I ran into an issue that only showed up on CI because i had the user default set, so bin/test didn't actually use the old link resolver and covered up an issue.

[d-ronnqvist]

What do you think about having the environment variable override the user default here? I ran into an issue that only showed up on CI because i had the user default set, so bin/test didn't actually use the old link resolver and covered up an issue.

I think that's a good idea. There's still the chance that someone would set both to different values and get confused by the result but I think it's less likely that someone would forget an environmental variable than a user default.

We could do something more sophisticated—like warning if both configuration values conflict—but the plan is to remove this configuration after Swift 5.9 so I don't think that doing more than changing the order would be worth it here.

[d-ronnqvist]

@swift-ci please test