Set BOOST_ROOT to BOOST_ROOT_1_69_0 in Azure Windows job

One of the recent updates to the agent images used on Azure removed BOOST_ROOT variable: actions/runner-images@e91f213, however it is needed by the CMake Boost finder scripts.

.ci/azure-pipelines/build-windows.yaml

@@ -22,6 +22,7 @@ jobs:
       - script: set
         displayName: 'Print Environment Variables'
       - script: |
+          echo ##vso[task.setvariable variable=BOOST_ROOT]%BOOST_ROOT_1_69_0%
           echo ##vso[task.prependpath]%BOOST_ROOT%\lib
         displayName: 'Update System PATH'
       - script: |