Fix osu cross-compilation

pkgs/mpich/default.nix

@@ -47,7 +47,7 @@ in mpich.overrideAttrs (old: {
 
   preFixup = ''
     sed -i 's:^CC=.*:CC=${targetPackages.stdenv.cc}/bin/${targetPackages.stdenv.cc.targetPrefix}cc:' $out/bin/mpicc
-    sed -i 's:^CXX=.*:CXX=${targetPackages.stdenv.cc}/bin/${targetPackages.stdenv.cc.targetPrefix}c++:' $out/bin/mpicxx
+    sed -i 's:^CXX=.*:CXX=${targetPackages.stdenv.cc}/bin/${targetPackages.stdenv.cc.targetPrefix}++:' $out/bin/mpicxx
   '' + lib.optionalString enableFortran ''
     sed -i 's:^FC=.*:FC=${targetPackages.gfortran or gfortran}/bin/${targetPackages.gfortran.targetPrefix or gfortran.targetPrefix}gfortran:' $out/bin/mpifort
   '';

pkgs/osu/default.nix

@@ -34,7 +34,7 @@ stdenv.mkDerivation rec {
 
   env = {
     MPICH_CC="${stdenv.cc}/bin/${stdenv.cc.targetPrefix}cc";
-    MPICH_CXX="${stdenv.cc}/bin/${stdenv.cc.targetPrefix}c++";
+    MPICH_CXX="${stdenv.cc}/bin/${stdenv.cc.targetPrefix}++";
   };
 
   postInstall = ''