Fix CI

[SkySkimmer]

No description provided.

[SkySkimmer]

(hopefully)

[andrew-appel]

Did we intend to use COMCPERT=bundled in VST's own CI? I discussed the idea that Coq's CI for VST used "bundled", but I believe we intend that VST's own CI should get CompCert from opam.

[SkySkimmer]

compcert 3.10 doesn't have a opam package so not sure what the alternative is

[Zimmi48]

The remaining issue, which is specific to 8.14 and the 32bit version, seems like a real one:

 - File "./sha/general_lemmas.v", line 56, characters 34-53:
 - Error: Found no subterm matching "?M3114 mod ?M3115" in the current goal.

As for getting CompCert 3.10 from opam, I'd rather defer it to @MSoegtropIMC to add the package, since he has done it in the past and best knows what to do.

[andrew-appel]

In other PRs, some of which are already in the master branch, we are fixing the CI in a different way.