Try to please new Pixi

Author: paugier

plugins/pixi.toml

@@ -22,6 +22,6 @@ openmpi = ">=4.1.6,<4.2"
 fluidfft-builder = ">=0.0.1,<0.1"
 mpi4py = ">=3.1.5,<3.2"
 pip = ">=24.0,<25"
-fftw = ">=3.3.10,<3.4 mpi*"
+fftw = {version = ">=3.3.10,<3.4", build="mpi*"}
 pytest = ">=8.0.0,<8.1"
 pkg-config = ">=0.29.2,<0.30"