[ci] Fix for MKL container builds.

Follow-up on !5011.

scripts/cmake/GuixSetup.cmake

@@ -18,8 +18,9 @@ endif()
 
 set(GUIX_BUILD ON CACHE BOOL "" FORCE)
 set(OGS_BUILD_TESTING OFF CACHE BOOL "" FORCE) # TODO: not yet supported
-# TODO: create a newer eigen package:
-set(OGS_USE_EIGEN_UNSUPPORTED OFF CACHE BOOL "" FORCE)
+# When building with OGS_USE_MKL then add guix build parameter for newer eigen,
+# e.g.: --with-commit=eigen=9000b3767770f6dd0f4cfb12f4e19c71921885a4
+set(OGS_USE_EIGEN_UNSUPPORTED ${OGS_USE_MKL} CACHE BOOL "" FORCE)
 set(OGS_INSTALL_DEPENDENCIES OFF CACHE BOOL "" FORCE) # handled by guix
 set(OGS_CPU_ARCHITECTURE OFF CACHE BOOL "" FORCE) # enables guix --tune
 set(BUILD_SHARED_LIBS ON CACHE BOOL "" FORCE)