FormulaInstaller: flush before closing writes.

This seems to help the problem with `brew install -v` sometimes truncating output.

Closes Homebrew/homebrew#24666.

Library/Homebrew/build.rb

@@ -46,6 +46,7 @@ def main
 rescue Exception => e
   unless error_pipe.nil?
     e.continuation = nil if ARGV.debug?
+    error_pipe.flush
     Marshal.dump(e, error_pipe)
     error_pipe.close
     exit! 1

Library/Homebrew/formula_installer.rb

@@ -371,6 +371,7 @@ class FormulaInstaller
         read.close
         exec(*args)
       rescue Exception => e
+        write.flush
         Marshal.dump(e, write)
         write.close
         exit! 1
@@ -378,6 +379,7 @@ class FormulaInstaller
     end
 
     ignore_interrupts(:quietly) do # the fork will receive the interrupt and marshall it back
+      write.flush
       write.close
       Process.wait
       data = read.read