nixos/man-db: allow man-cache to be fetched from cache

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.

nixos/modules/misc/man-db.nix

@@ -52,7 +52,7 @@ in
     environment.systemPackages = [ cfg.package ];
     environment.etc."man_db.conf".text =
       let
-        manualCache = pkgs.runCommandLocal "man-cache" { } ''
+        manualCache = pkgs.runCommand "man-cache" { } ''
           echo "MANDB_MAP ${cfg.manualPages}/share/man $out" > man.conf
           ${cfg.package}/bin/mandb -C man.conf -psc >/dev/null 2>&1
         '';