Wrong Date format in version output

[mastrolinux]

Bug Report

The date output is not a RFC3339 format

Current behavior

Build date does not have an easy parseable date format like RFC3339
now the CLI output is:

./arduino-cli version --format json | jq .build_date
"2019-07-16 15:06:29.7368222 +0000 UTC"

Expected behavior

it was supposed to use: https://golang.org/pkg/time/#Time.Format
it should print something RFC3339 compatible, not a custom Go language format

Environment

• CLI version (output of arduino-cli version):
  {
  "command": "arduino-cli",
  "version": "0.3.7-alpha.preview",
  "commit": "",
  "build_date": "2019-07-16 15:09:59.2173451 +0000 UTC"
  }
• Go version (if building from sources):
  1.12.5
• OS version:
  Windows and Linux

Additional context

This was needed to improve command line testing.