ci: clear the checkout token in every job that does not push (#5547)

actions/checkout writes its token into .git/config as an http extraheader,
where it stays for the rest of the job. That matters more here than usual:
pull_request jobs run on self-hosted runners and execute the PR's own build.rs,
proc-macros and tests, any of which can read that file. Test and Lint holds
actions: write on top of that, so its token can cancel runs and delete the
Actions caches the whole pipeline now depends on — it was given
persist-credentials: false when that permission was added, and this extends the
same treatment to the other 45 checkouts.

Two are exempt because the token IS the credential the job needs.
helm-package's publish job pushes to rustfs/helm with it; clearing it would
break chart publishing. nix-flake-update is exempt pending verification: it
passes FLAKE_UPDATE_TOKEN to create-pull-request directly rather than reusing
.git/config, so it very likely does not need persistence, but that is unproven
and a broken weekly bot is not worth the guess. Both carry a
persist-credentials-exempt comment saying which.

scripts/security/check_persist_credentials.sh requires every checkout to either
clear its credentials or carry that comment, so the decision stays visible in
review rather than being an omission nobody notices.

Refs: rustfs/backlog#1598, rustfs/backlog#1602

.github/workflows/helm-package.yml

@@ -50,6 +50,8 @@ jobs:
     steps:
       - name: Checkout helm chart repo
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
+        with:
+          persist-credentials: false
 
       # Both inputs reach the shell through env rather than `${{ }}`
       # interpolation. A git ref name may contain `$(...)` — anything without a
@@ -121,6 +123,8 @@ jobs:
       - name: Checkout helm package repo
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
         with:
+          # persist-credentials-exempt: this checkout's token IS the push credential —
+          # the job git-pushes to rustfs/helm below. Clearing it breaks chart publishing.
           repository: rustfs/helm
           token: ${{ secrets.RUSTFS_HELM_PACKAGE }}