Currently, we build `man-cache` with `runCommandLocal`, which causes it
to get re-built locally instead of fetched from cache. While the
resulting derivation might be small, it does take a fair bit of time to
build for all my systems, and would be _far_ quicker to fetch.
With this change, we use `runCommand` instead of `runCommandLocal`,
allowing it to get fetched from cache instead of rebuilt for all hosts.
39e63f110e