Add support for building and testing with flambda to the new CI script
Add support for building and testing with flambda to the new CI script

If the flambda environment variable is set to true, the feature is
enabled in the build system.

(cherry picked from commit 24b028c1fc433e46772ec19ec7adc5e2cc4d0100)

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml