Fix CI build on Windows. (#1495)

.github/workflows/continuous.yml

@@ -111,7 +111,9 @@ jobs:
 
         # https://github.com/actions/cache/issues/101
       - name: Set env
-        run: echo "::set-env name=appdata::$($env:LOCALAPPDATA)"
+        run: |
+          echo "::set-env name=appdata::$($env:LOCALAPPDATA)"
+          echo "::set-env name=BOOST_ROOT::$env:BOOST_ROOT_1_69_0"
 
       - name: Cache build
         id: cache-build

.github/workflows/nightly.yml

@@ -149,6 +149,7 @@ jobs:
     env:
       CC: cl.exe
       CXX: cl.exe
+      BOOST_ROOT: ${{ env.BOOST_ROOT_1_69_0 }}
     strategy:
       fail-fast: false
       matrix:
@@ -166,6 +167,10 @@ jobs:
           fetch-depth: 10
       - uses: seanmiddleditch/gha-setup-ninja@master
 
+      - name: Set env
+        run: |
+          echo "::set-env name=BOOST_ROOT::$env:BOOST_ROOT_1_69_0"
+
         # We run configure + build in the same step, since they both need to call VsDevCmd
         # Also, cmd uses ^ to break commands into multiple lines (in powershell this is `)
       - name: Configure and build