dartdev prints analyticsNoticeOnFirstRunMessage when CI=true

@jwren, This is what I suggested we fix in #43017 (comment):

[~]$rm -rf .dart/
[~]$CI=true /source/flutter/bin/dart --help
  ╔════════════════════════════════════════════════════════════════════════════╗
  ║ The Dart tool uses Google Analytics to anonymously report feature usage    ║
  ║ statistics, and crash reporting to send basic crash reports. This data is  ║
  ║ used to help improve the Dart platform and tools over time.                ║
  ║                                                                            ║
  ║ To disable reporting of anonymous tool usage statistics in general, run    ║
  ║ the command: `dart --disable-analytics`.                                   ║
  ╚════════════════════════════════════════════════════════════════════════════╝

We do not record any analytics when the env var CI=true.
But it would be nice, if we did not print the analyticsNoticeOnFirstRunMessage when CI=true.

Notably, this is likely to break tests in some CI environments 🙂