crane: update test to avoid Docker Hub (#69330)

Docker Hub has a very low IP-based rate limit:
https://docs.docker.com/docker-hub/download-rate-limit/

To avoid hitting that limit, we'll point this `crane manifest` test at
`gcr.io/go-containerregistry/crane`, as GCR has much more generous rate
limiting. Further, the gcr.io/go-containerregistry/crane repository is
under the control of the authors of crane, so we will be able to
anticipate any issues with this test.

I've removed the mediaType check from this test, as it is likely to
break in the future when we migrate to OCI-based media types. The
schemaVersion MUST be 2 for both docker manifests and OCI manifests, so
this test should continue working for quite a while.

Formula/crane.rb

@@ -22,9 +22,8 @@ class Crane < Formula
   end
 
   test do
-    json_output = shell_output("#{bin}/crane manifest homebrew/brew")
+    json_output = shell_output("#{bin}/crane manifest gcr.io/go-containerregistry/crane")
     manifest = JSON.parse(json_output)
     assert_equal manifest["schemaVersion"], 2
-    assert_equal manifest["config"]["mediaType"], "application/vnd.docker.container.image.v1+json"
   end
 end