locate: build in correct dbpath

nixos/modules/misc/locate.nix

···
         }
       ];
 
+    nixpkgs.config = { locate.dbfile = cfg.output; };
+
     environment.systemPackages = [ cfg.locate ];
+
+    environment.variables = mkIf (!isMLocate)
+      { LOCATE_PATH = cfg.output;
+      };
 
     warnings = optional (isMLocate && cfg.localuser != null) "mlocate does not support searching as user other than root"
             ++ optional (isFindutils && cfg.pruneNames != []) "findutils locate does not support pruning by directory component"

nixos/modules/programs/environment.nix

···
   config = {
 
     environment.variables =
-      { LOCATE_PATH = "/var/cache/locatedb";
-        NIXPKGS_CONFIG = "/etc/nix/nixpkgs-config.nix";
+      { NIXPKGS_CONFIG = "/etc/nix/nixpkgs-config.nix";
         PAGER = mkDefault "less -R";
         EDITOR = mkDefault "nano";
       };

pkgs/tools/misc/mlocate/default.nix

···
-{ stdenv, fetchurl }:
+{ stdenv, fetchurl, config }:
 
-stdenv.mkDerivation rec {
+let
+  dbfile = stdenv.lib.attrByPath [ "locate" "dbfile" ] "/var/cache/locatedb" config;
+in stdenv.mkDerivation rec {
   name = "mlocate-${version}";
   version = "0.26";
 
···
   };
 
   buildInputs = [ ];
+  makeFlags = [ "dbfile=${dbfile}" ];
 
   meta = with stdenv.lib; {
     description = "Merging locate is an utility to index and quickly search for files";