Draft: fix buildPebbleApp, use new SDK, test reverse dependencies compilation

[t184256]

This currently fails in CI with

error: hash mismatch in fixed-output derivation '/nix/store/9kirksnyvp4p4khsy88q5nbaykaicll5-stpyv8-13.1.201.22-cp312-cp312-manylinux_2_31_x86_64.whl.drv':
           likely URL: https://files.pythonhosted.org/packages/cp312/s/stpyv8/stpyv8-13.1.201.22-cp312-cp312-manylinux_2_31_x86_64.whl
            specified: sha256-g0uXYbt/SdqLiHhHx2R0laLPbEX2niEkrg4/AkSTvBU=
                  got: sha256-wkqkIVxk231n/GxCwNdzHKvPMAWWv5yCaudPQm/jt3E=
        expected path: /nix/store/92jsnc2aknw8sm6kx60dl7s7mbbqhfaw-stpyv8-13.1.201.22-cp312-cp312-manylinux_2_31_x86_64.whl
             got path: /nix/store/hayxzqiwa72ci9kqazy2lxhwjpksz5s6-stpyv8-13.1.201.22-cp312-cp312-manylinux_2_31_x86_64.whl

for reasons that evade me.

I don't insist things should be done this way, treat it more like a list of things that I think should be done for others to use your flake and feel safe doing it: feel free to cherry-pick, amend, etc.

[Sorixelle]

Yeah, I've definitely gotta go back and look at that function again - been meaning to give it some attention, I've just been more focused on the development setup experience lately. Having some proper tests for it is definitely a good idea though, I'll take a look at this PR when I get around to doing that. Thanks!