Rather than bother with custom groovy to hack the Jenkins declarative pipeline environment, let's use a structural paradigm that has been successful for me because of its simplicity (the only cost being a reusable custom groovy function) and readability (the function wrapper demonstrates what it does in salutary clarity).

int withVSEnvironment(String label, Boolean returnStatus, String script) {
    ret = powershell(
        label: label,
        returnStatus: returnStatus,
        script: """
            foreach (\$_ in cmd /c "`"%VS140COMNTOOLS%`"..\\..\\vc\\bin\\vcvars32.bat > nul 2>&1 & SET") {
                if (\$_ -match '^([^=]+)=(.*)') {
                    [System.Environment]::SetEnvironmentVariable(\$matches[1], \$matches[2])
                }
            }

            $script
        """
    )
    if (returnStatus) {
        return ret
    }
}


pipeline {
    agent any
    stages {
        stage('Stage Left') {
            steps {
                withVSEnvironment(
                    'Building with PowerShell!!!',
                    False,  // returnStatus
                    '''
                    # No weak cmd script here
                    MSBuild.exe Source/Answer/The_Solution.sln /m /t:Clean,Rebuild
                    '''
                )
            }
        }
    }
}