Refactor Deadline field of codersdk.WorkspaceBuild to *time.Time

This was implemented as a `time.Time` but would be nicer for the UI to consume as a `*time.Time`.

> 🙈 I should probably have made that type a `*time.Time` for you -- we can refactor that in a follow up PR.

_Originally posted by @johnstcn in https://github.com/coder/coder/pull/1825#discussion_r883360485_