Use expanded path for --with-opt-dir

.github/workflows/windows.yml

@@ -166,10 +166,11 @@ jobs:
         # windows-2019 image doesn't have OpenSSL as of 2023/9/14
         if: ${{ matrix.vs != 2019 }}
 
+      # TODO: We should use `../src` instead of `D:/a/ruby/ruby/src`
       - name: Configure
         run: >-
           ../src/win32/configure.bat --disable-install-doc
-          --with-opt-dir=../src/vcpkg_installed/x64-windows
+          --with-opt-dir=D:/a/ruby/ruby/src/vcpkg_installed/x64-windows
 
       - run: nmake incs