[do not merge]: new bootstrap tools with local test

[ghost]

Description of changes

take changes from

• freshBootstrapTools: remove overlay, prune tools, use tar.xz archive #295557
  plus a few minor modifications so that it is possible to build the tools locally and then use them to bootstrap. need to find aarch64 testers.

1. get the changes

   git fetch https://github.com/NixOS/nixpkgs.git pull/296201/head:bootstrap-test
   git switch bootstrap-test
   

2. with sandbox enabled, build bootstrap tools and then bootstrap stdenv on aarch64-darwin (need help with this as do not have aarch64)

   nix-build  pkgs/top-level/release.nix -A stdenvBootstrapTools.aarch64-apple-darwin.stdenv
   

3. (i've done this on big-sur vm) with sandbox enabled, build bootstrap tools and then bootstrap stdenv on x64-darwin

    nix-build  pkgs/top-level/release.nix -A stdenvBootstrapTools.x86_64-apple-darwin.stdenv
   

4. cleanup -- delete the branch

   git switch -
   # delete the branch
   git branch -D bootstrap-test
   

[ghost]

@philiptaron could you help with testing this on aarch64-darwin if your machine has a day or two of free cycles? :-)
with sandbox enabled.
i found a machine.

[philiptaron]

Ah, I'm at a retreat for this weekend with only a few computer minutes (a good thing!). I take it by the closure of this PR that the test came back badly?

[ghost]

Ah, I'm at a retreat for this weekend with only a few computer minutes (a good thing!). I take it by the closure of this PR that the test came back badly?

enjoy! tests are succeeding. this change is just a hacked up version changes #295557 and #295558 to enable testing with locally built tools.

closed the PR so i wouldn't send pings if i made updates.