Fix RBE jobs (#4032)
Hopefully this fixes this error we're now seeing on jobs.
```
(17:55:08) ERROR: /var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/ec321eb2cc2d0f8f91b676b6d4c66c29/external/bazel_ci_rules+/rbe_repo.bzl:64:13: An error occurred during the fetch of repository '+rust_test+buildkite_config':
Traceback (most recent call last):
File "/var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/ec321eb2cc2d0f8f91b676b6d4c66c29/external/bazel_ci_rules+/rbe_repo.bzl", line 64, column 13, in _rbe_preconfig_impl
fail("\nrbe_preconfig: Unsupported toolchain '{}' (Values: {}).\n".format(toolchain_name, _available_toolchain_names(manifest)))
Error in fail:
rbe_preconfig: Unsupported toolchain 'ubuntu1804-bazel-java11' (Values: 'ubuntu2404').
(17:55:08) ERROR: com.google.devtools.build.lib.packages.RepositoryFetchException: no such package '@@+rust_test+buildkite_config//config':
rbe_preconfig: Unsupported toolchain 'ubuntu1804-bazel-java11' (Values: 'ubuntu2404').
```
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