Use "pre" instead of "nightly" for GitHub CI

[gdalle]

Partial fix for #474

Changing the LTS is slightly more involved so let's start with avoiding nightly

[codecov[bot]]

Codecov Report

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

Project coverage is 94.33%. Comparing base (759bb41) to head (bbf0fac).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #475 +/- ## ======================================= Coverage 94.33% 94.33% ======================================= Files 24 24 Lines 742 742 ======================================= Hits 700 700 Misses 42 42 ```

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