Fix JSON end interval receiver seconds value

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master 3.21+

• Issues fixed (if any): Upstream receiver end intervals reporting wrong seconds value #836

• Brief description of code changes (suitable for use as a commit message):
  end_time was used for the "end:streams:receiver:seconds" value instead of receiver_time.

[bmah888]

Thanks for the PR! I haven't tested this yet but it'd be super-cool if this was the simple fix to a long-standing bug.