Fix secondary Xcode schemes ignoring action_config_names settings

Created by: rockbruno

Fixes https://github.com/facebook/buck/issues/2265

We use the following setup to create a secondary scheme that sends Enterprise builds:

xcode_workspace_config(
    name = "workspace",
    workspace_name = "MyApp",
    src_target = ":MyApp",
    environment_variables = ENV_VAR,
    extra_schemes = {
      'MyApp-Enterprise': ':myapp_enterprise_workspace',
    }
)

xcode_workspace_config(
    name = "myapp_enterprise_workspace",
    workspace_name = "MyApp-Enterprise",
    src_target = ":MyApp",
    environment_variables = ENV_VAR,
    action_config_names = {"archive": "Release-Enterprise"},
)

However, setting action_config_names on the secondary scheme currently has no effect because the schemes were always pointing to the main scheme's configs.

This PR fixes this to make secondary schemes pull their configs from their own definition.

Note that this will affect projects that were relying on this to have all schemes having the same configs and has the side effect that project scheme's actions won't be equal to workspaceArguments anymore - should we keep the old interaction if custom configs weren't provided?