project-lakitu: Check if grub.cfg exist before trying to change permission

grub.cfg doesn't exist in the ARM image.

BUG=b/179398937
TEST=`cuskyctl builds submit lakitu` and local lakitu-arm64 build
RELEASE_NOTE=None

Change-Id: I87b17136dc672b3451d024830dbd25b2e200848d
Reviewed-on: https://cos-review.googlesource.com/c/cos/overlays/board-overlays/+/12070
Reviewed-by: Roy Yang <royyang@google.com>
Reviewed-by: Oleksandr Tymoshenko <ovt@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
1 file changed