Reduce effective scope of pipe variables

Library/Homebrew/formula.rb

@@ -493,8 +493,6 @@ class Formula
   # Pretty titles the command and buffers stdout/stderr
   # Throws if there's an error
   def system cmd, *args
-    rd, wr = IO.pipe
-
     # remove "boring" arguments so that the important ones are more likely to
     # be shown considering that we trim long ohai lines to the terminal width
     pretty_args = args.dup
@@ -510,6 +508,9 @@ class Formula
     logfn = "#{logd}/%02d.%s" % [@exec_count, File.basename(cmd).split(' ').first]
     mkdir_p(logd)
 
+    rd, wr = IO.pipe
+
+    begin
       pid = fork { exec_cmd(cmd, args, rd, wr, logfn) }
       wr.close
 
@@ -544,6 +545,7 @@ class Formula
     ensure
       rd.close unless rd.closed?
     end
+  end
 
   private