[gitlab/ci] Prevent Corn from running if Bignums has failed.
[gitlab/ci] Prevent Corn from running if Bignums has failed.

The needs keyword needs to include all the transitive dependencies.
Otherwise, we risk stage-3 being skipped but stage-4 being run as in
https://gitlab.com/coq/coq/pipelines/78119475.

The reason why we don't need to include bignums in the dependencies of
corn is because the artifact for math-classes is the directory where
both bignums and math-classes were built.

SkySkimmer/coq,silene/coq,coq/coq,JasonGross/coq,gares/coq,SkySkimmer/coq,Zimmi48/coq,Zimmi48/coq,SkySkimmer/coq,gares/coq,gares/coq,silene/coq,Zimmi48/coq,silene/coq,coq/coq,Matafou/coq,gares/coq,JasonGross/coq,Zimmi48/coq,ejgallego/coq,Matafou/coq,Matafou/coq,coq/coq,ejgallego/coq,JasonGross/coq,ejgallego/coq,coq/coq,JasonGross/coq,ejgallego/coq,SkySkimmer/coq,gares/coq