Remove specific gcc version for installing ``pynbody``

[jobovy]

Doesn't seem to be necessary currently because we are back to installing pynbody wheels and when building pynbody becomes necessary again, might have to do GCC13 anyway..

[codecov[bot]]

Codecov Report

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

Project coverage is 99.91%. Comparing base (1745307) to head (0d98a5a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #647 +/- ## ======================================= Coverage 99.91% 99.91% ======================================= Files 200 200 Lines 29260 29260 Branches 563 564 +1 ======================================= Hits 29236 29236 Misses 24 24 ```

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