use gz

tools/pylib/_boutcore_build/build-wheels.sh

 set -ex
 yum -y install fftw-devel netcdf-cxx-devel
-curl https://github.com/dschwoerer/multimpi/releases/download/v0.1/multimpi-0.1.tar.xz > multimpi.tar.xz
-tar -xvf multimpi.tar.xz -C /usr/local
+curl https://github.com/dschwoerer/multimpi/releases/download/v0.1/multimpi-0.1.tar.xz > multimpi.tar.gz
+tar -xvf multimpi.tar.gz -C /usr/local
 MPIPATH=/usr/local/multimpi
 export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$MPIPATH/lib/
 ./configure --enable-shared  CC=$MPIPATH/bin/mpicc CXX=$MPIPATH/bin/mpic++