Fix windows header path.

Author: stefanseefeld

.github/workflows/test-windows.yml

@@ -42,8 +42,8 @@ jobs:
     - name: build
       shell: cmd
       run: |
-        faber --builddir=build cxx.name=msvc --with-boost-include=${{ runner.workspace }}/vcpkg/installed/x64-windows/include -j4
+        faber --builddir=build cxx.name=msvc --with-boost-include=${{ runner.workspace }}\vcpkg\installed\x64-windows\include -j4
     - name: test
       shell: cmd
       run: |
-        faber --builddir=build cxx.name=msvc --with-boost-include=${{ runner.workspace }}/vcpkg/installed/x64-windows/include -j4 test.report
+        faber --builddir=build cxx.name=msvc --with-boost-include=${{ runner.workspace }}\vcpkg\installed\x64-windows\include -j4 test.report