build.cmd does not terminate MSBuild.exe processes after completion

#### Background
My harddrive died so I had to reinstall everything including Visual Studio and the msbuild project, when I was installing VS I thought I don't need `.NET Framework v3.5` so I unchecked it.

And `msbuild.cmd` failed because of MSB3645, where the v3.5 SDK is needed. I tried to re-install the SDK using VS installer, but then it reported `MSBuild.exe` is in use.

I wonder if MSBuild may have gotten left running after the build script failed, so I tested it and this seems to happen consistently.

__I thought I was done__, but as I was going to verify this issue for a clean run (run script without error), turns out the MSBuild processes did not shutdown even when the build succeeded.

Is this normal or expected?

### Steps to reproduce
1. Run `Build.cmd` from Developer Command Prompt for VS 2017.
1. Expect it to fail due to `error MSB3645: .NET Framework v3.5 Service Pack 1 was not found.`
1. Close the command prompt and open up Task Manager to verify `MSBuild.exe` is Running

### Expected behavior (?)
`MSBuild.exe` should not appear in task manager as Running after `Build.cmd` finished running (?)

### Actual behavior
`MSBuild.exe` is up and running.

#### OS info:
`Microsoft Windows [Version 10.0.17134.254]`
`Microsoft Visual Studio Community 2017
Version 15.8.3`
`Microsoft .NET Framework
Version 4.7.03056`