tests: ops: use proper imports

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

tests/ops/fetch-container/nonexistent/image-with-valid-digest/main.cue

@@ -1,10 +1,11 @@
 package testing
 
+import "dagger.io/dagger/op"
+
 // XXX WATCHOUT
 // Once buildkit has pulled that digest, it will stay cached and happily succeed WHATEVER the image name then is
 #up: [
-	{
-		do:  "fetch-container"
+	op.#FetchContainer & {
 		ref: "busyboxaaa@sha256:e2af53705b841ace3ab3a44998663d4251d33ee8a9acaf71b66df4ae01c3bbe7"
 	},
 ]