Flake lock file updates:

• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/b550fe4b4776908ac2a861124307045f8e717c8e' (2024-02-28)
  → 'github:NixOS/nixpkgs/88dca77be222aedd1f47d2cf0942dffefee76216' (2024-06-03)

Manually tested by printing to stderr in both branches (sp in os
stack, or not), and triggering a GC in a filterSource function,
e.g.:

    let
      generateTree = n: if n == 0 then "ha" else { left = generateTree (n - 1); right = generateTree (n - 1); };
    in
      builtins.deepSeq (generateTree 18) ...

Note that the darwin still uses the strategy of disabling GC, despite
having an implementation that compiles. The proper solution will be
enabled and tested later.

This re-enables support for older bwdgc versions without complicating
the code too much.
Coroutines generally only interfere with GC during source filtering,
so it's not too bad of a regression on older bdwgc.
This seems preferable over conditional compilation to enable the patch
etc; we've already spent a lot of complexity budget on this GC-coroutine
interaction...

Nixpkgs has reshuffled its Nix versions.

Temporarily(?) blocked on NixOS/nixpkgs#297475 (comment)