[infra] Always produce a proper Dart version string on release channels
[infra] Always produce a proper Dart version string on release channels

Previously, if --custom_for_pub was used we used the latest dev tag.
This "best effort" for Dart rolls from bleeding edge, but doesn't make
sense when rolling from other channels that have a proper version in the
VERSION file.

Change-Id: Icaf6c26858d9fd2132d26b7d64da21b3a05f0092
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/105763
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>

dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk