test: fix the test

Signed-off-by: Ramkumar Chinchani <[email protected]>

test/basic.bats

@@ -323,6 +323,12 @@ EOF
     # /ccc is created in l2, removed and recreated in l3
     # /ddd is created in l1, removed and recreated in l2, and extended in l3
     cat > stacker.yaml <<"EOF"
+busybox:
+    from:
+        type: oci
+        url: ${{BUSYBOX_OCI}}
+    run: |
+        touch /foo
 l1:
     from:
        type: tar
@@ -351,7 +357,7 @@ l3:
        mkdir -p /ccc/222/ab
        mkdir -p /ddd/333/ab
 EOF
-    stacker build
+    stacker build --substitute BUSYBOX_OCI=${BUSYBOX_OCI}
     umoci unpack --image oci:l3 l3
 
     # aaa/111 should be deleted, aaa/222 should exist