Blocks plugin calls $block->build() twice

Issue Summary

In Drupal\context\Plugin\ContextReaction\Blocks:execute() this code is used to build the block and add it's contextual links to the block #theme element render array.

// Add contextual links to block.
$content = $block->build();
if (isset($content['#contextual_links'])) {
    $blockBuild['#contextual_links'] = $content['#contextual_links'];
}

I don't quite understand the need for this as it is now invoking the block->build() twice while ::preRenderBlock actually renders the block.

I don't see this double-build issue in any other modules ex:
Drupal\block\BlockViewBuilder::buildPreRenderableBlock
Drupal\ds\Plugin\DsField\BlockBase::build

Proposed Resolution

Remove the $block->build() from Drupal\context\Plugin\ContextReaction\Blocks:execute().