fix the means by which cmake finds the suitesparse include directory

[superwhiskers]

this is just a really minor fix so that cmake finds the include directory for suitesparse (on my system, at least)

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.

[superwhiskers]

the test failure does not appear to be related to the change this pull request is for

[nkoukpaizan]

the test failure does not appear to be related to the change this pull request is for

There's a non-deterministic behavior somewhere. I just re-ran the pipeline and it passed. The wrong answer we got when it failed was kind of scary.

[superwhiskers]

i saw it fail in #169 as well. same test if i recall correctly, PhasorDynamicsGenClassicalTest