fftwLongDouble: 3.3.7 -> 3.3.8 (#41299)

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/fftw-long-double/versions.

These checks were done:

- built on NixOS

- 0 of 0 passed binary check by having a zero exit code.
- 0 of 0 passed binary check by having the new version present in output.
- found 3.3.8 with grep in /nix/store/7csndk7q4h2wvv46ndgh6lm4j0ybx9zf-fftw-long-double-3.3.8
- directory tree listing: https://gist.github.com/05084878f5535b72e17b90667821c468
- du listing: https://gist.github.com/174653df406b155bf1a9c07da9257f54