Benchmark workflow fails during actions/checkout@v6 with "could not read Username for 'https://github.com'"

Description

The benchmark workflow fails during actions/checkout@v6, before any benchmark code is executed.

The failing step is:

- uses: actions/checkout@v6
  with:
    persist-credentials: false

The log shows that checkout creates a $RUNNER_TEMP credentials config file and adds includeIf.gitdir entries, but the following git fetch still fails:

/usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +a0d8219bafaec98ac9a5d1f21416ccef7a07df73:refs/remotes/pull/12144/merge
Error: fatal: could not read Username for 'https://github.com': terminal prompts disabled

This seems unrelated to the PR's Rust changes, because the job fails before the repository is checked out.

This may be related to the credential handling changes in actions/checkout@v6, where credentials are stored in a separate $RUNNER_TEMP config file and referenced via includeIf.gitdir.

A similar upstream issue exists here:

https://github.com/actions/checkout/issues/2359

It is not exactly the same scenario, since that issue mainly discusses Docker container actions, while this failure happens during checkout's own git fetch. However, the root cause appears similar: the v6 credential setup is created, but Git does not successfully use it for authentication.

Start with the benchmark workflow's actions/checkout@v6 step and reproduce the failing git fetch from the supplied log. Compare the $RUNNER_TEMP credentials config and includeIf.gitdir behavior with upstream actions/checkout issue #2359; done means checkout authenticates successfully and the workflow reaches the benchmark code.