Authenticate CI git fetches of GitHub repos

[samuelburnham]

GitHub throttles anonymous git HTTPS traffic per source IP, and Warp runners share egress IPs across tenants. Starting 2026-09-02 ~12:30 UTC that pool was throttled, so unauthenticated clones of public repos began failing with 401s ("could not read Username for 'https://github.com'"): Nix's eval-time builtins.fetchGit of Lake manifest dependencies broke Nix CI, and Lake's own dependency clone broke cuda-compile, across unrelated PRs.

Add an authenticate-github-fetches composite action that rewrites github.com URLs via url.insteadOf to carry the job's ephemeral GITHUB_TOKEN, moving every git-CLI fetch (Lake dependencies, eval-time fetchGit, script clones) off the shared anonymous IP pool and onto per-token limits. Wire it into:

• setup-rust-toolchain, alongside CARGO_NET_GIT_FETCH_WITH_CLI=true so cargo's git dependencies fetch through the git CLI and pick up the rewrite (cargo's built-in fetcher ignores insteadOf);
• both nix.yml jobs, which don't use that wrapper (install-nix-action's github_access_token covers only github: flake inputs, not git+https fetchGit);
• the bench compile jobs in bench-main and bench-pr, whose lake build Compile<env> clones mathlib and friends without the wrapper.

In bench-pr this puts the token in ~/.gitconfig where checked-out PR code can read it, which persist-credentials: false previously kept off disk. That is acceptable: the workflow's permissions block pins the token to read-only on a public repo (the dispatch job's actions:write never runs PR code), so the exposure matches an ordinary pull_request run. Toolchain-only lean-action jobs (build: false) clone nothing and are left alone, as is CodeQL, which manages its own fetches.