minimal-bootstrap: use builtins.fetchGit

[ghost]

Description of changes

Experiment to try to avoid needing an upload to tarballs.nixos.org.

I'm building this now. It is sloooooow.

This evals for me:

nix build -f '<nixpkgs>' minimal-bootstrap.stage0-posix.src \
  --system i686-linux \
  --option restrict-eval true \
  --option allowed-uris https://github.com/oriansj/stage0-posix

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[ghost]

Now I remember why this can't work: #232576 (comment)