Set CLICOLOR_FORCE=0

Since ocaml#6340 we're considering `CLICOLOR_FORCE` to determine whether
stderr supports color. However this changes behavior observable from the
test suite, so `CLICOLOR_FORCE=1 make test` fails (this is tracked as
ocaml#6607).

The issue is that ocaml/setup-ocaml#631, this variable is set in the CI
environment. So we disable it until the situation is fixed (and this
variable does not have observable changes anymore).

Signed-off-by: Etienne Millon <[email protected]>

dune-file

@@ -46,4 +46,5 @@
 
 (env
  (_
-  (flags :standard -alert -unstable)))
+  (flags :standard -alert -unstable)
+  (env-vars (CLICOLOR_FORCE 0))))