File _multibuild of Package imb

<multibuild>
  <package>openmpi4</package>
  <package>openmpi5</package>
  <package>mpich</package>
  <package>mvapich2</package>
  <package>mvapich3-ucx</package>
  <package>mvapich3-ofi</package>
  <package>mvapich4-ucx</package>
  <package>mvapich4-ofi</package>
</multibuild>