Pkg.jl installs very outdated version of FFTW.jl

[ranocha]

We had some CI issues like

Error building `FFTW`: 
ERROR: LoadError: Unable to open libLLVM!
[...]

(see https://github.com/trixi-framework/Trixi.jl/actions/runs/12906499717/job/35987944652#step:7:56) caused by Pkg.jl installing a very old version of FFTW.jl (version 1.1.1 from 2019) which used BinaryProvider.jl - which is not maintained anymore and which does not work on many new systems. The reason for this seemed to be that recent versions of FFTW.jl depend on MKL_jll but did not allow the 2025 version of MKL_jll (until JuliaMath/FFTW.jl#314). FFTW.jl v1.1.1 was the last version which didn't even depend on MKL_jll.

Thus, upgrading FFTW.jl from v1.1.1 to v1.8.0 lead Pkg.jl to downgrade to jlls:

  [621f4979] ↑ AbstractFFTs v0.5.0 ⇒ v1.5.0
  [b99e7846] - BinaryProvider v0.5.10
  [8f4d0f93] - Conda v1.10.2
  [7a1cc6ca] ↑ FFTW v1.1.1 ⇒ v1.8.0
  [81def892] - VersionParsing v1.3.0
  [f5851436] + FFTW_jll v3.3.10+3
⌅ [1d5cc7b8] ↓ IntelOpenMP_jll v2025.0.4+0 ⇒ v2024.2.1+0
⌅ [856f044c] ↓ MKL_jll v2025.0.1+0 ⇒ v2024.2.0+0

@vchuravy suggested to open an issue here - quoting him:

I think it's a bi-stable solution point in the package resolver. I would open an issue on Pkg.jl