Build the standard environment packages from the stable Nixpkgs

[thomasjm]

The Bash kernel, both spellchecker language servers, and the two testing packages were all built
from pkgsMaster. Each one forces the master Nixpkgs tree to be fetched and realised, which costs
466 MB and ~90k filesystem entries in every sandboxed package store that evaluates them.

None of them needed master:

• testing.builds-forever / testing.builder-uid take only runCommand. (Their module versions
  already used config.pkgs; only the top-level attrs in codedown.nix were on master.)
• The spellcheckers need nodehun, nodejs-slim, hunspell, hunspellDicts and unixtools, all
  present in stable. Fixed in both places that build them: the top-level spellchecker attr that
  packageSearch includes, and the module that environments resolve
  language-servers.spellchecker to.
• The Bash kernel moved to master in 52196ee because bash_kernel's test suite failed on macOS on
  release-25.05. Stable is now release-25.11 and it builds cleanly there, so it goes back to
  config.pkgs and the stale note about the workaround comes out.

With these, the whole standard package set -- kernels.bash, kernels.python3,
language-servers.spellchecker, shells.bash, shells.zsh -- no longer touches master.

Testing

Evaluated every affected package with pkgsMaster = throw "MASTER FORCED", so any remaining
reference fails loudly; all come back clean, including the standard package set as a whole.

Built on both platforms:

• kernels.bash on x86_64-linux and on aarch64-darwin (bash_kernel-0.10.0 through
  pythonImportsCheckPhase)
• both spellcheckers, and each answers an LSP initialize over stdio with its capabilities, so the
  nodehun native module loads

./.aliases/dev-verify-default-nix passes.

Related

The pypy3 kernel work that was originally on this branch is now #100, and the pylint test flake is
#99. Both are independent of this change.

Note that nix flake check fails on main today, on lspProbeEnvs, for a reason unrelated to this
PR; #100 is what fixes that.