Remove "build" tag on Dune from OPAM file

It's not recommended for Dune, see
ocaml/opam-repository#14185.

Author: bbc2

zxcvbn.opam

@@ -13,7 +13,7 @@ run-test: [
   ["dune" "runtest" "-p" name "-j" jobs]
 ]
 depends: [
-  "dune" {build & >= "1.4.0"}
+  "dune" {>= "1.4.0"}
   "ocaml" {>= "4.04.0"}
   "ounit" {with-test}
   "ppx_deriving" {>= "4.0" & < "5.0"}