Formula#system: sanitize args before exec

This prevents passing nested arrays to exec; the same thing is done in
safe_system.

Fixes Homebrew/homebrew#10295.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/formula.rb

@@ -448,6 +448,7 @@ protected
         rd.close
         $stdout.reopen wr
         $stderr.reopen wr
+        args.collect!{|arg| arg.to_s}
         exec(cmd, *args) rescue nil
         exit! 1 # never gets here unless exec threw or failed
       end