FortranCInterface: Fix wrong path when deciding if redetection is needed

d7c80305 (FortranCInterface: Fix misuse of IS_NEWER_THAN in timestamp check, 2021-02-21) updated the IS_NEWER_THAN logic, but it introduced a couple of errors. 2a00e507 (FortranCInterface: Fix regression in timestamp check, 2021-09-30) addressed one of those errors, but there was still one left behind that wasn't noticed. The Output.cmake file is in the build directory, but there was still one reference to it that incorrectly used a path to it in the source directory.

Issue: #22709 (closed)