How to pass env-var to MSBuild Task and make it effective for all sub-process? (concrete code)

Question

I have a question about passing environment variables to MSBuild Task. See my code below:

a.proj

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003" 
    DefaultTargets="A1">
    
    <Target Name="A0">
        <Message Text="A0 start."/>
        <MSBuild Projects="b.proj" Properties="myvar=000" />
        <Message Text="A0 end."/>
    </Target>

    <Target Name="A1" DependsOnTargets="A0">
        <Message Text="A1 start."/>
        <MSBuild Projects="b.proj" Properties="myvar=111" />
        <Message Text="A1 end."/>
    </Target>
</Project>

b.proj

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    <Target Name="B">
        <Message Text="B start."/>
        <Exec command="c.bat" />
    </Target>
</Project>

c.bat

echo [C]myvar=%myvar%

I hope that c.bat can see myvar's value passed from a.proj. My code above failed to do that.

Consider(Assume) a running <MSBuild> Task a process on the OS, then, I'd like to pre-set environment variable myvar for that process, so that its subprocesses(no matter how deep) can all see myvar's value.

How can I achieve that? Thank you.