FortranCInterface: Fix regression in timestamp check

Since commit d7c8030541 (FortranCInterface: Fix misuse of IS_NEWER_THAN
in timestamp check, 2021-02-21, v3.21.0-rc1~631^2~3), FortranCInterface
checks for `Output.cmake.in` in the build tree instead of the source
tree as before.  This caused it to always re-run the detection.

Fixes: #22709