Merge branch 'fix/CI_4.13' into 'master'

Forces CI to select correct opam cache.

See merge request laiser/caisar!20

.gitlab-ci.yml

@@ -10,7 +10,8 @@ tests:
       - _opam
   script:
   - if [ ! -d _opam ]; then echo "no local switch in the CI cache, we setup a new switch"; opam switch create --yes --no-install . ocaml-base-compiler.4.13.1; fi
-  - eval $(opam env)
+  - opam switch 4.13
+  - eval $(opam env --switch=4.13 --set-switch)
   - opam switch
   - sudo apt-get update
   - sudo apt install -y protobuf-compiler