Redirect stderr into build.log in Windows pipeline

The build.log is created and found, but no warnings are collected.
Redirect stderr so the warnings end up in the log in the first place.

Fixes: #8159
Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

build/jenkins/pipelines/windows.Jenkinsfile

@@ -108,7 +108,7 @@ pipeline {
                                 steps {
                                     powershell """
                                         cd build\\workspaces\\vs2017
-                                        & \"${visualStudioPath}\" pyrogenesis.sln /p:Configuration=${BUILD_TYPE} ${buildOptions} \$env:JOBS | Tee-Object -FilePath ${BUILD_TYPE}-build.log
+                                        & \"${visualStudioPath}\" pyrogenesis.sln /p:Configuration=${BUILD_TYPE} ${buildOptions} \$env:JOBS 2>&1 | Tee-Object -FilePath ${BUILD_TYPE}-build.log
                                     """
                                 }
                                 post {