Test custom PETSc path

[jkozdon]

#117 add ability to have a custom PETSc path. Not sure the best way to test this.

We could manually set against the petsc library from the binary builder by setting something like DYLD_LIBRARY_PATH?

[ViralBShah]

The issue is that PETSc_jll will always be installed. It may be easy to test on linux, where you install the package manager version and try it out, but it has to be done carefully.