Fix a find_package(UMFPACK) bug in ParU/CMakeLists.txt

[valgur]

External UMFPACK is currently not being found due to a small bug in ParU/CMakeLists.txt.

[mmuetzel]

Oof. That's probably a copy-paste error that wasn't caught by CI because CAMD_FOUND happened to be set after find_package(CHOLMOD).

Your change looks good to me. Thank you for your contribution. 👍

Edit: Strike that previous remark. The CI didn't catch it because it is linking to the CHOLMOD libraries in the common build or source tree. 🤦
Your change still looks good though. 👍

[DrTimothyAldenDavis]

Thanks for the catch.

I realize this is a very minor change, but my organization requires that any contributor signs a contributor agreement (a copy is at the top-level in SuiteSparse). Can you sign it and send it to me?

[mmuetzel]

This change fixes a bug in the current CMake rules. I'm assuming that @valgur didn't respond in more than 6 weeks. With that in mind, what should be done with this change?

[valgur]

I emailed my signed contributor license now. Apologies for the delay.