Ensure plugin directory exists, otherwise checkout lock thrashes

[lox]

It appears that with #827 we've introduced a bug where if the plugin directory doesn't exist the agent will never be able to acquire the checkout lock. The result is races with output like:

# Could not acquire lock on "/usr/local/var/buildkite-agent/plugins/github-com-buildkite-plugins-docker-buildkite-plugin-v1-4-0.lock" (open /usr/local/var/buildkite-agent/plugins/953740525: no such file or directory)
--
  | # Trying again in 1s...
  | # Could not acquire lock on "/usr/local/var/buildkite-agent/plugins/github-com-buildkite-plugins-docker-buildkite-plugin-v1-4-0.lock" (open /usr/local/var/buildkite-agent/plugins/969463144: no such file or directory)
  | # Trying again in 1s...
  | # Could not acquire lock on "/usr/local/var/buildkite-agent/plugins/github-com-buildkite-plugins-docker-buildkite-plugin-v1-4-0.lock" (open /usr/local/var/buildkite-agent/plugins/800153511: no such file or directory)

This PR fixes the issue by ensuring that the PluginsPath exists prior to acquiring the lock.