CI: make docpreview pass for pull requests

[pjonsson]

Reason for this pull request

The docpreview job fails for me on a pull request where I didn't update any documentation. Bring in the changes from datacube-ows (https://github.com/opendatacube/datacube-ows/pull/972, https://github.com/opendatacube/datacube-ows/pull/974) that I think fixed the same issue.

Proposed changes

• [ ] Closes #xxxx
• [ ] Tests added / passed
• [ ] Fully documented, including docs/about/whats_new.rst for all changes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.78%. Comparing base (81b4a31) to head (6fd8e8a). Report is 3 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1625 +/- ## ======================================== Coverage 91.78% 91.78% ======================================== Files 132 132 Lines 14640 14640 ======================================== Hits 13437 13437 Misses 1203 1203 ```

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