chore(nixery): use nix-1p from within the depot

Since the source of nix-1p is checked in under //nix/nix-1p, we should
use it from there if Nixery is being built inside of depot.

Change-Id: Iddd54f7b93b398b2f909db6ee105366a9914a2ac
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5882
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>

nix/nix-1p/default.nix

@@ -4,8 +4,9 @@
 # This file configures TVL CI to mirror the subtree to GitHub.
 { depot ? { }, pkgs ? import <nixpkgs> { }, ... }:
 
-(pkgs.runCommandLocal "nix-1p.md" { } ''
-  cp ${./README.md} $out
+(pkgs.runCommandLocal "nix-1p" { } ''
+  mkdir $out
+  cp ${./README.md} $out/README.md
 '').overrideAttrs (_: {
   meta.ci.extraSteps.github = depot.tools.releases.filteredGitPush {
     filter = ":/nix/nix-1p";