Fix msbuild main codeflow

[dkurepa]

Msbuild codeflow is currently blocked and they needed a branch reset.
The first step in fixing it is done dotnet/msbuild#12709
Now we need a new build from this commit, and then need to forward flow it, and then finally backflow
I blocked the currently open backflow dotnet/msbuild#12721 (review)

[dkurepa]

blocked on a failing msbuild, will reach out to them

[dkurepa]

We have a FF dotnet/dotnet#3231.
After it's merged we should close the currently open backflow and re trigger the sub