[CI] Remove coq-bignums before CI

[proux01]

Otherwise the test are run with the bignums already in the Docker image rather than the one just built by the CI.

Cc @erikmd (could you check that this looks reasonnable?) @ppedrot (following #86 )

[proux01]

@erikmd I'm lost with the permissions in the Docker image here, could you have a look?

[erikmd]

Thanks @proux01, indeed the opam remove -y coq-bignums before the pinning was the way to go.
However to avoid permission issues, I suggest not directly relying on make, but still using opam commands.

[proux01]

Fixed, thanks @ppedrot for having noticed that stupid issue.