chore(nix): move buildManPages to //users/sterni

It's only used in my user directory and I don't feel it's generally
useful. There's so little to interpolate you may as well just check them
in.

Change-Id: I31dc9917c2b281c7d68388e1a32c8ef5179621df
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12873
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

nix/buildManPages/OWNERS

@@ -1 +0,0 @@
-sterni

users/sterni/dot-time-man-pages/default.nix

@@ -65,6 +65,6 @@ let
   };
 
 in
-depot.nix.buildManPages "dot-time" { } [
+depot.users.sterni.nix.build.manPages "dot-time" { } [
   dot-time
 ]