cmd: make use of maps.{Copy, Clone}

cmd: make use of maps.{Copy, Clone}

Change-Id: I8a38b4c71c34d3544ee32be9c6e767bb1099a720
GitHub-Last-Rev: ff4cb4e91be3936465635f99d061f02999640ed9
GitHub-Pull-Request: golang/go#69424
Reviewed-on: https://go-review.googlesource.com/c/go/+/612735
Reviewed-by: Keith Randall <khr@golang.org>
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>