Build failure: python313 --enable-experimental-jit

Steps To Reproduce

I'm attempting to build Python 3.13.0b4 with the new experimental JIT. However supplying the configure flag breaks the build, noting that it cannot find python3. Just removing the --enable-experimental-jit is enough for it to build correctly.

Steps to reproduce the behaviour:

Grab the flake.nix from the build logs below
Note it overrides configureFlags adding "--enable-experimental-jit" and adds the required clang_18 and llvm_18 as mentioned here.
nix build .#python313_JIT with attached flake

Build log

Here's the build log, flake.nix, and flake.lock of a plain python313 build failure when enabling the experimental JIT.

Additional context

This seems to stem from this line within the configure.ac. It seems the PYTHON_FOR_REGEN is not being correctly found.

I can build 3.13.0b4 from source outside of nix with a default configuration and --enable-experimental-jit.

Supplying the build with pkgs.python313 as a buildInput or nativeBuildInputs resolves this particular issue but only leads to a rather uninformative KeyError: 6. Relevant gist here. KeyError is on line 2835.

No change on unstable.

Notify maintainers

No maintiners listed in the meta.maintainers but @mweinelt and @tjni are listed as the Python team members.

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

[user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
this path will be fetched (0.00 MiB download, 0.00 MiB unpacked):
  /nix/store/9j1f45904jshhlsmb9ljqaxdkg86ajl2-nix-info
copying path '/nix/store/9j1f45904jshhlsmb9ljqaxdkg86ajl2-nix-info' from 'https://cache.nixos.org'...
 - system: `"x86_64-linux"`
 - host os: `Linux 6.9.9-1-default, openSUSE Tumbleweed, noversion, nobuild`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.23.3`
 - nixpkgs: `/nix/store/73jd5krdaw6r94zwfra7bi6qxw4lr0dp-source`

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