Stop testing on nightly

[andreasnoack]

There are so many failures that it makes the CI status useless. We should trigger CI upcoming Julia releases in dedicated PRs instead of unconditionally testing on nightly.

[codecov-commenter]

Codecov Report

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

Project coverage is 85.90%. Comparing base (13029c0) to head (df17fbb). Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1861      +/-   ##
==========================================
- Coverage   85.99%   85.90%   -0.09%     
==========================================
  Files         144      144              
  Lines        8666     8655      -11     
==========================================
- Hits         7452     7435      -17     
- Misses       1214     1220       +6     

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[devmotion]

I think this PR can be closed since the problem was already fixed by #1863 and #1888.