Unexport CMD_DURATION

Valid uses of this environment variable don't really include passing
it to subsequent child processes.

I confirmed the fix with:

  function fish_prompt
      echo "cmd duration [$CMD_DURATION] "
  end

  cmd duration [0] sleep 2
  cmd duration [2002]

1 files changed, 1 insertions, 1 deletions

diff --git a/reader.cpp b/reader.cpp
index ba50cd98..b2f2b4a6 100644
--- a/reader.cpp
+++ b/reader.cpp
@@ -2508,7 +2508,7 @@ void set_env_cmd_duration(struct timeval *after, struct timeval *before)
     }
 
     swprintf(buf, 16, L"%d", (secs * 1000) + (usecs / 1000));
-    env_set(ENV_CMD_DURATION, buf, ENV_EXPORT);
+    env_set(ENV_CMD_DURATION, buf, ENV_UNEXPORT);
 }
 
 void reader_run_command(parser_t &parser, const wcstring &cmd)