[flang][cmake] Fix variable reference (${var} instead of var)

This is used for out-of-tree Flang builds when FLANG_BUILD_NEW_DRIVER is
set.

GitOrigin-RevId: 2f8e614df8832abde3d123bed8b67f901b7b6966
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1318eaf..812a794 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -57,7 +57,7 @@
       CLANG_DIR_ABSOLUTE
       ${CLANG_DIR}
       REALPATH
-      CMAKE_CURRENT_SOURCE_DIR)
+      ${CMAKE_CURRENT_SOURCE_DIR})
     list(APPEND CMAKE_MODULE_PATH ${CLANG_DIR_ABSOLUTE})
 
     # TODO: Remove when libclangDriver is lifted out of Clang