[dartdev] Use underscore for output dir name of dart build
Follow-up to https://dart-review.googlesource.com/c/sdk/+/433160.
This aligns us with how `dart --version` and `Abi.current` prints the
OS/architecture pair.
Cq-Include-Trybots: luci.dart.try:pkg-linux-debug-try,pkg-linux-release-arm64-try,pkg-linux-release-try,pkg-mac-release-arm64-try,pkg-win-release-arm64-try,pkg-mac-release-try,pkg-win-release-try
Bug: https://github.com/dart-lang/sdk/issues/60730
Change-Id: I8c56da29e5cd7c6222a7c5ed7e685501a279030f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/435123
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Michael Goderbauer <goderbauer@google.com>
Michael Goderbauer
77d75fc735