The error is in this line
<MSBuild Projects="$(MSBuildProjectFile)" Targets="CustomDeployMagicFirmware" Properties="Id=1" />
The MSBuild task spawns a new MSBuild.exe process and you are not passing the property $(BuildNumber) to the new process. Change it like below and this would work
<MSBuild Projects="$(MSBuildProjectFile)" Targets="CustomDeployMagicFirmware" Properties="Id=1;BuildNumber=$(BuildNumber)" />