[make setup]: do not automatically invoke [make -C fuzz setup].

Makefile

@@ -43,12 +43,19 @@ all:
 # [make setup] installs the packages that we need in whatever opam
 # switch is currently active.
 
+# Installing ocamlfind and monolith in the current switch is not
+# necessary for [make] nor [make test]; but it is necessary for
+# [make -C fuzz setup].
+
+# We do not invoke [make -C fuzz setup] here, because it is costly
+# (it can create a new switch) and is useful only to people who
+# wish to run [make test].
+
 .PHONY: setup
 setup:
 	opam install \
 	  "dune>=2.0" "cppo" "pprint>=20200410" "seq" "odoc" \
 	  "monolith" "ocamlfind"
-	make -C fuzz setup
 
 .PHONY: merlin
 merlin: