ruby: disable parallel install

Without the change parallel installs fail as:

    installing rdoc:                    ...-ruby-2.7.7-devdoc/share/ri/2.7.0/system
    Traceback (most recent call last):
            13: from ./tool/rbinstall.rb:954:in `<main>'
            12: from ./tool/rbinstall.rb:954:in `each'
            11: from ./tool/rbinstall.rb:957:in `block in <main>'
            10: from ./tool/rbinstall.rb:438:in `block in <main>'
             9: from ./tool/rbinstall.rb:262:in `install_recursive'
             8: from ./tool/rbinstall.rb:262:in `each'
             7: from ./tool/rbinstall.rb:271:in `block in install_recursive'
             6: from ./tool/rbinstall.rb:174:in `install'
             5: from /build/ruby-2.7.7/lib/fileutils.rb:876:in `install'
             4: from /build/ruby-2.7.7/lib/fileutils.rb:1587:in `fu_each_src_dest'
             3: from /build/ruby-2.7.7/lib/fileutils.rb:1605:in `fu_each_src_dest0'
             2: from /build/ruby-2.7.7/lib/fileutils.rb:1589:in `block in fu_each_src_dest'
             1: from /build/ruby-2.7.7/lib/fileutils.rb:882:in `block in install'
    /build/ruby-2.7.7/lib/fileutils.rb:882:in `chmod': No such file or directory @ apply2files - ...-ruby-2.7.7-devdoc/share/ri/2.7.0/system/ARGF/inspect-i.ri (Errno::ENOENT)
    make: *** [uncommon.mk:373: do-install-all] Error 1
    make: *** Waiting for unfinished jobs....

pkgs/development/interpreters/ruby/default.nix

@@ -95,6 +95,10 @@ let
         propagatedBuildInputs = op jemallocSupport jemalloc;
 
         enableParallelBuilding = true;
+        # /build/ruby-2.7.7/lib/fileutils.rb:882:in `chmod':
+        #   No such file or directory @ apply2files - ...-ruby-2.7.7-devdoc/share/ri/2.7.0/system/ARGF/inspect-i.ri (Errno::ENOENT)
+        # make: *** [uncommon.mk:373: do-install-all] Error 1
+        enableParallelInstalling = false;
 
         patches = op (lib.versionOlder ver.majMin "3.1") ./do-not-regenerate-revision.h.patch
           ++ op (atLeast30 && useBaseRuby) (