Fix DMD path to ../generated

[wilzbach]

Cherry-picked #5892 for the stable branch.

From braddr/at-client#4 (comment)

Sorry, not staging, but stable. All tested branches need to have the changes. In fact, since I merged the auto-tester changes in before remembering to check stable, it's currently failing to build. That needs to be fixed asap or I'll have to revert the auto-tester or put in a temporary work around. It'd be best to just get the changes merged.

[dlang-bot]

Thanks for your pull request, @wilzbach!

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.