Put constraint.txt where python build can find it.

.github/workflows/windows.yml

@@ -213,12 +213,12 @@ jobs:
           core.exportVariable('VCPKG_BINARY_SOURCES', "clear;x-gha,readwrite");
           core.exportVariable('VCPKG_ROOT', process.env.VCPKG_INSTALLATION_ROOT || '');
 
-    - name: Setup PIP_CONSTRAINTS (workaround https://github.com/python-cffi/cffi/issues/117)
+    - name: Setup PIP_CONSTRAINT (workaround https://github.com/python-cffi/cffi/issues/117)
       if: matrix.options.py == 'ON' && matrix.check_mkvk != 'ONLY'
       run: |
-          mkdir $env:BUILD_DIR # Doesn't exist until CMake configure
-          echo 'setuptools<74' > $env:BUILD_DIR/constraint.txt
-          echo "PIP_CONSTRAINT=$env:BUILD_DIR/constraint.txt" >> $env:GITHUB_ENV
+          mkdir interface/python_binding/build # Doesn't exist until python build.
+          echo 'setuptools<74' > interface/python_binding/build/constraint.txt
+          echo "PIP_CONSTRAINT=interface/python_binding/build/constraint.txt" >> $env:GITHUB_ENV
 
     - name: Install Dependencies
       if: matrix.check_mkvk != 'ONLY'