Selecting a Swiftly toolchain always sets it globally

[matthewbastien]

Describe the bug
The toolchain selection workflow asks whether or not I want to install the toolchain in User settings or Workspace settings. However, it always updates the global Swiftly toolchain regardless of which option I pick.

To Reproduce
Steps to reproduce the behavior:

1. Run the Swift: Select Toolchain... command.
2. When prompted, select any toolchain installed via Swiftly.
3. When prompted, select any Xcode toolchain for the DEVELOPER_DIR.
4. When prompted, select Workspace Configuration for the location to install the toolchain.

The DEVELOPER_DIR gets set correctly in workspace settings. However, only the Swiftly global toolchain is updated.

Expected behavior
The .swift-version file should be created/modified in my workspace to match the version I selected in the dialog.

[matthewbastien]

Completed with #1860.