[flang][driver] Revert RUN-line change

In https://reviews.llvm.org/D98283, the RUN line in pre-fir-tree04.f90
was updated to use `%flang_fc1` instead of `%f18` (so that the test is
shared between the old and the new driver). Unfortunately, the new
driver does not know yet how to find standard intrinsics modules. As a
result, the test fails when `FLANG_BUILD_NEW_DRIVER` is set to On.

I'm restoring the original RUN line. This is rather straightforward, so
sending without a review. This should make Flang builders happy.