Add dart install bin directory to PATH #181

Problem

dart install (Dart 3.10+) is the recommended replacement for dart pub global activate, but it installs executables into the Dart data-home install directory—not $PUB_CACHE/bin.

Today, setup-dart adds these directories to PATH:

• $DART_HOME/bin (SDK)
• $PUB_CACHE/bin (legacy dart pub global activate)

It does not add the dart install bin directory. After migrating CI workflows from dart pub global activate melos to dart install melos, subsequent steps fail with:

melos: command not found

dart install itself prints a warning that the install directory is not on PATH:

Warning: Dart installs executables into $HOME/.local/state/Dart/install/bin, which is not on your path.

On GitHub Actions, setup-dart is typically the step that configures PATH for the job, so users expect globally installed Dart CLI tools to work in later steps without manual $GITHUB_PATH workarounds.