sw:vec_iree: Fix HAL driver setting (trial 2)

Need to force update and cache IREE_HAL_DRIVERS_TO_BUILD to match
https://github.com/google/iree/blob/a547965/CMakeLists.txt#L53

Change-Id: I314b975e259f1f946775843122301318c08e005a

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 00cd065..98b7f64 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -43,7 +43,7 @@
 set(IREE_ENABLE_THREADING OFF)
 set(IREE_BUILD_TESTS OFF)
 set(IREE_BUILD_BINDINGS_TFLITE OFF)
-set(IREE_HAL_DRIVERS_TO_BUILD "Dylib_Sync;VMVX_Sync")
+set(IREE_HAL_DRIVERS_TO_BUILD "Dylib_Sync;VMVX_Sync" CACHE STRING "IREE HAL Drivers" FORCE)
 
 #-------------------------------------------------------------------------------
 # IREE-specific definitions