Set ECM_PREFIX at build time rather than at runtime
Since cmake commit https://gitlab.kitware.com/cmake/cmake/-/commit/6ddf8712cdcc1b548af4b502ea91e94cf0bf4f0b the package prefix variable is non-deterministic
Since cmake commit https://gitlab.kitware.com/cmake/cmake/-/commit/6ddf8712cdcc1b548af4b502ea91e94cf0bf4f0b the package prefix variable is non-deterministic