Fix metrics job

It was failing because the pager wasn't set, causing the message

  error: executing '': No such file or directory

to pollute the JSON output.

https://hydra.nixos.org/build/154768851

Issue #76776.

pkgs/top-level/metrics.nix

@@ -9,6 +9,7 @@ runCommand "nixpkgs-metrics"
   }
   ''
     export NIX_STATE_DIR=$TMPDIR
+    export NIX_PAGER=
     nix-store --init
 
     mkdir -p $out/nix-support
@@ -24,13 +25,11 @@ runCommand "nixpkgs-metrics"
         # Redirect stdout to /dev/null to avoid hitting "Output Limit
         # Exceeded" on Hydra.
         nix-env.qaDrv|nix-env.qaDrvAggressive)
-          NIX_SHOW_STATS=1 time -o stats-time "$@" 2>stats-nix >/dev/null ;;
+          NIX_SHOW_STATS=1 NIX_SHOW_STATS_PATH=stats-nix time -o stats-time "$@" >/dev/null ;;
         *)
-          NIX_SHOW_STATS=1 time -o stats-time "$@" 2>stats-nix ;;
+          NIX_SHOW_STATS=1 NIX_SHOW_STATS_PATH=stats-nix time -o stats-time "$@" ;;
       esac
 
-      sed '/^warning:/d' -i stats-nix
-
       cat stats-nix; echo; cat stats-time; echo
 
       x=$(jq '.cpuTime' < stats-nix)