[firehose] Should use the SDK specified in pubspec for validation

[dcharkes]

The SDK version can be specified for the firehose invocation:

https://github.com/dart-lang/ecosystem/tree/main/pkgs/firehose#publishing-from-a-specific-version-of-the-sdk

However, this doesn't work very well with mono repos, one might have a single package that tracks a dev version of the SDK.

Instead, firehose should use the version from the pubspec.

If that is technically impractical (due to the Dart SDK version being passed into some GitHub action setup), maybe we should have a best practice that we have a health configuration per package instead?

Context: Starting to track a dev branch of the Dart SDK for a single package in a mono repo:

• Support @Native fields and addressOf native#860