Run the breakage test against a ponyc that still moves

shared-docker renamed the pcre builder image and moved it off the `latest`
tag on 2025-10-20. The old name kept resolving, so nothing failed and the
workflow went on building against the ponyc frozen into that image.

Closes #70

Author: SeanTAllen

.github/workflows/breakage-against-ponyc-latest.yml

@@ -12,7 +12,7 @@ jobs:
     name: Test against ponyc main
     runs-on: ubuntu-latest
     container:
-      image: ghcr.io/ponylang/shared-docker-ci-x86-64-unknown-linux-builder-with-pcre:latest
+      image: ghcr.io/ponylang/shared-docker-ci-standard-builder-with-pcre:nightly
     steps:
       - uses: actions/checkout@v6.0.2
       - name: Test