Repair CI for OS X

[michael-schwarz]

The CI for Mac currently fails with message opam: unknown command depext'.`.

Constantly having failing tests means it is easy to miss if another test newly fails.

[vogler]

Some notes:

macos: opam/2.1.0 https://github.com/goblint/cil/runs/3874800641#step:4:1010
ubuntu: opam/2.0.8 https://github.com/goblint/cil/runs/3874800980#step:4:61

https://opam.ocaml.org/blog/opam-2-1-0/#Seamless-integration-of-System-dependencies-handling-a-k-a-quot-depexts-quot

cil is still using the v1 action:

cil/.github/workflows/tests.yml

Line 46 in c16dddf

uses: avsm/setup-ocaml@v1

goblint is using the v2 action:
https://github.com/goblint/analyzer/blob/0f0ffcd4b4b2292b1525a5c17aadd330d66faced/.github/workflows/unlocked.yml#L39

[vogler]

Fixed by upgrading to setup-ocaml@v2 (b1f8137) which uses opam/2.0.9 on macos.

Previous comments on v1 vs. v2 (mainly caching): goblint/analyzer#296

[vogler]

Before 33b13e2 all but 4.04.2 worked.
After, all macos suddenly fail: Package zarith is already installed https://github.com/goblint/cil/runs/3875924530#step:9:121
Finicky CI... Maybe some issue with the cache.

[vogler]

Fixed by 24ffb6c which removed the old cache action (setup-ocaml@v2 caches as well).