ocaml: disable parallel installs

Without the change parallel install fails as:

    $ install flags: -j16 ...
    ...
    install: target '...-ocaml-4.14.0/lib/ocaml/threads': No such file or directory
    make[1]: *** [Makefile:140: installopt] Error 1

pkgs/development/compilers/ocaml/generic.nix

@@ -87,6 +87,11 @@ stdenv.mkDerivation (args // {
   #  make[2]: *** [Makefile:199: backup] Error 1
   enableParallelBuilding = lib.versionAtLeast version "4.08";
 
+  # Workaround missing dependencies for install parallelism:
+  #  install: target '...-ocaml-4.14.0/lib/ocaml/threads': No such file or directory
+  #  make[1]: *** [Makefile:140: installopt] Error 1
+  enableParallelInstalling = false;
+
   # Workaround lack of parallelism support among top-level targets:
   # we place nixpkgs-specific targets to a separate file and set
   # sequential order among them as a single rule.