Set MATH_LIBRARY_BACKEND in publish workflow

Add MATH_LIBRARY_BACKEND environment variable to CI workflow

Author: pariterre

.github/workflows/publish_eigen_to_pypi.yml

@@ -57,6 +57,7 @@ jobs:
         with:
           config-file: "pyproject.toml"
         env:
+          MATH_LIBRARY_BACKEND: ${{ env.MATH_LIBRARY_BACKEND }}
           CIBW_SKIP: "pp*-manylinux_i686"
           CIBW_BEFORE_BUILD: >-
             rm -rf _skbuild