[codex] Address some more GHA hygiene issues (#21622)

This does two things:

- We use `persist-credentials: false` everywhere now. This is
unfortunately not the default in GitHub Actions, but it prevents
`actions/checkout` from dropping `secrets.GITHUB_TOKEN` onto disk.
- We interpose (some) template expansions through environment variables.
I've limited this to contexts that have non-fixed values; contexts that
are fixed (like `*.result`) are not dangerous to expand directly inline
(but maybe we should clean those up in the future for consistency
anyways).

This is a medium-risk change in terms of CI breakage: I did a scan for
usage of `git push` and other commands that implicitly use the persisted
credential, but couldn't find any. Even still, some implicit usages of
the persisted credentials may be lurking. Please ping ww@ if any issues
arise.

.github/workflows/codespell.yml

@@ -19,6 +19,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
       - name: Annotate locations with typos
         uses: codespell-project/codespell-problem-matcher@b80729f885d32f78a716c2f107b4db1025001c42 # v1.1.0
       - name: Codespell