build-llvm: Set CMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ONLY when cross compiling

This avoids picking up a host cmake config file for zstd, in the github
actions CI, in the current git main version of llvm.

build-llvm.sh

@@ -194,6 +194,7 @@ if [ -n "$HOST" ]; then
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY"
+    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ONLY"
 
     # Custom, llvm-mingw specific defaults. We normally set these in
     # the frontend wrappers, but this makes sure they are enabled by