Use both alt-ergo 2.4.2 and 2.5.3

Author: belolourenco

.github/workflows/set-up-environment.yml

@@ -20,6 +20,9 @@ jobs:
         opam update
         opam install why3.1.6.0
         opam install alt-ergo.2.4.2
+        eval $(opam env)
+        cp $(which alt-ergo) $(which alt-ergo).2.4.2
+        opam install alt-ergo.2.5.3
         opam install eprover.2.6
         # it's possible to install z3 as follows
         # opam install z3.4.11.2