git clone for PRs fails after upgrade to Buildkite Agent 3.23.1

Log: https://buildkite.com/bazel/bazel-bazel-github-presubmit/builds/6941#07cd8d84-f495-4f45-a4e8-5e110509ed5e

```
# Creating "/var/lib/buildkite-agent/builds/bk-docker-pblz/bazel/bazel-bazel-github-presubmit"
--
  | $ cd /var/lib/buildkite-agent/builds/bk-docker-pblz/bazel/bazel-bazel-github-presubmit
  | # Using git-mirrors experiment 🧪
  | $ cd /var/lib/gitmirrors
  | # Updating existing repository mirror to find commit 6eb25d3aee16bd6d5bff4de291bb5852a471dd5a
  | $ git --git-dir /var/lib/gitmirrors/https---github-com-bazelbuild-bazel-git remote set-url origin https://github.com/bazelbuild/bazel.git
  | $ git --git-dir /var/lib/gitmirrors/https---github-com-bazelbuild-bazel-git fetch origin katre:order-proto-outputs
  | fatal: Couldn't find remote ref katre
  | ⚠️ Warning: Checkout failed! exit status 128 (Attempt 1/3 Retrying in 2s)
  | # Removing /var/lib/buildkite-agent/builds/bk-docker-pblz/bazel/bazel-bazel-github-presubmit
  | # Creating "/var/lib/buildkite-agent/builds/bk-docker-pblz/bazel/bazel-bazel-github-presubmit"
  | $ cd /var/lib/buildkite-agent/builds/bk-docker-pblz/bazel/bazel-bazel-github-presubmit
  | # Using git-mirrors experiment 🧪
  | $ cd /var/lib/gitmirrors
  | # Updating existing repository mirror to find commit 6eb25d3aee16bd6d5bff4de291bb5852a471dd5a
  | $ git --git-dir /var/lib/gitmirrors/https---github-com-bazelbuild-bazel-git remote set-url origin https://github.com/bazelbuild/bazel.git
  | $ git --git-dir /var/lib/gitmirrors/https---github-com-bazelbuild-bazel-git fetch origin katre:order-proto-outputs
  | fatal: Couldn't find remote ref katre
  | ⚠️ Warning: Checkout failed! exit status 128 (Attempt 2/3 Retrying in 2s)
  | # Removing /var/lib/buildkite-agent/builds/bk-docker-pblz/bazel/bazel-bazel-github-presubmit
  | # Creating "/var/lib/buildkite-agent/builds/bk-docker-pblz/bazel/bazel-bazel-github-presubmit"
  | $ cd /var/lib/buildkite-agent/builds/bk-docker-pblz/bazel/bazel-bazel-github-presubmit
  | # Using git-mirrors experiment 🧪
  | $ cd /var/lib/gitmirrors
  | # Updating existing repository mirror to find commit 6eb25d3aee16bd6d5bff4de291bb5852a471dd5a
  | $ git --git-dir /var/lib/gitmirrors/https---github-com-bazelbuild-bazel-git remote set-url origin https://github.com/bazelbuild/bazel.git
  | $ git --git-dir /var/lib/gitmirrors/https---github-com-bazelbuild-bazel-git fetch origin katre:order-proto-outputs
  | fatal: Couldn't find remote ref katre
  | ⚠️ Warning: Checkout failed! exit status 128 (Attempt 3/3 Retrying in 2s)
  | # Removing /var/lib/buildkite-agent/builds/bk-docker-pblz/bazel/bazel-bazel-github-presubmit
  | # Creating "/var/lib/buildkite-agent/builds/bk-docker-pblz/bazel/bazel-bazel-github-presubmit"
  | $ cd /var/lib/buildkite-agent/builds/bk-docker-pblz/bazel/bazel-bazel-github-presubmit
  | 🚨 Error: exit status 128
```

Start with the linked Buildkite log and the checkout path using the git-mirrors experiment after the Buildkite Agent 3.23.1 upgrade. Inspect the failing `git fetch origin katre:order-proto-outputs` step and related CI configuration; done means PR checkouts complete successfully without the remote-ref failure.

Written by the indexing model from the issue text.