[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.

GitOrigin-RevId: 547624af1963f082f0c4676b0c019528372f6f13
diff --git a/test/Lower/pre-fir-tree04.f90 b/test/Lower/pre-fir-tree04.f90
index 55b0cb1..06fc88f 100644
--- a/test/Lower/pre-fir-tree04.f90
+++ b/test/Lower/pre-fir-tree04.f90
@@ -1,4 +1,4 @@
-! RUN: %flang_fc1 -fsyntax-only -fdebug-pre-fir-tree %s | FileCheck %s
+! RUN: %f18 -fsyntax-only -fdebug-pre-fir-tree %s | FileCheck %s
 
 ! Test Pre-FIR Tree captures all the coarray related statements