cirrus.yml: switch back to org docker image paths

These should work now that we have (automatically-updated) docker
builds via .github/workflows/docker_builds.yml.

Ref: https://github.com/fish-shell/fish-shell/pull/11884#discussion_r2405536855

.cirrus.yml

@@ -6,12 +6,12 @@ linux_task:
     matrix:
         - name: alpine
           container: &step
-              image: ghcr.io/krobelus/fish-ci/alpine:latest
+              image: ghcr.io/fish-shell/fish-ci/alpine:latest
               memory: 4GB
         - name: jammy
           container:
               <<: *step
-              image: ghcr.io/krobelus/fish-ci/jammy:latest
+              image: ghcr.io/fish-shell/fish-ci/jammy:latest
     tests_script:
         # cirrus at times gives us 32 procs and 2 GB of RAM
         # Unrestriced parallelism results in OOM