Add CIBW_ENVIRONMENT variable to workflow

Author: pariterre

.github/workflows/publish_eigen_to_pypi.yml

@@ -61,6 +61,7 @@ jobs:
           CIBW_SKIP: "pp*-manylinux_i686"
           CIBW_BEFORE_BUILD: >-
             rm -rf _skbuild
+          CIBW_ENVIRONMENT: MATH_LIBRARY_BACKEND=${{ env.MATH_LIBRARY_BACKEND }}
 
       - name: Store the wheels
         uses: actions/upload-artifact@v4