CI: run `opam update` before `opam install`

CI is failing as follows:

```
$ opam install coq-mathcomp-zify
[NOTE] It seems you have not updated your repositories for a while. Consider updating them with:
       opam update
The following dependencies couldn't be met:
  - coq-mathcomp-zify -> coq-mathcomp-algebra < 1.14~
      not available because the package is pinned to version 1.14.0
No solution found, exiting
```