Make sure we wait for the correct child

Fixes Homebrew/homebrew#27962.

Library/Homebrew/utils.rb

@@ -95,13 +95,13 @@ end
 module Homebrew
   def self.system cmd, *args
     puts "#{cmd} #{args*' '}" if ARGV.verbose?
-    fork do
+    pid = fork do
       yield if block_given?
       args.collect!{|arg| arg.to_s}
       exec(cmd.to_s, *args) rescue nil
       exit! 1 # never gets here unless exec failed
     end
-    Process.wait
+    Process.wait(pid)
     $?.success?
   end
 end