efi_loader: Disable miniapps on sandbox

In the sandbox environment we can not easily build efi stub binaries
right now, so let's disable the respective test cases for the efi
selftest suite.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>