Enable SCOTCH detection via CMake CONFIG mode.

[garth-wells]

Search first for SCOTCH in CONFIG mode (look for a SCOTCH-provided config file), and if not found search using MODULE mode (using the DOLFINx provided FindSCOTCH.cmake).

[minrk]

Makes sense! I think the only case #3241 handles that this doesn't is SCOTCH_DIR having highest priority when find_package(SCOTCH CONFIG) succeeds and would produce a different result. I'm not sure that's worth handling, but seems worth mentioning. With this, I think there is no configuration or override available to fix things if SCOTCHConfig doesn't produce the right links.

[minrk]

Just to confirm, I updated the conda-forge Windows build with this PR and it works fine.