docs workflow: try to fix it again

It's been broken since 514a009. Since 0e2d079,
the error was:

```
error: failed to push branch gh-pages to origin:
"fatal: could not read Username for 'https://github.com': No such device or address"
```

This is hard to test outside of prod, but follows a guess after
reading some Github docs and `gh auth login` docs from
<https://cli.github.com/manual/gh_auth_login>. I could not
find docs specifically about using the `git` CLI, but I'm guessing
that if `gh auth login` makes `git push` work on user machines by
setting `GH_TOKEN`, doing the same might help.

🤞

Cc @thoughtpolice, @martinvonz

.github/workflows/docs.yml

@@ -36,5 +36,7 @@ jobs:
           export MKDOCS_SITE_NAME="Jujutsu docs (prerelease)"
           export MKDOCS_PRIMARY_COLOR="blue grey"
           .github/scripts/docs-build-deploy prerelease --push
+        env:
+            GH_TOKEN:  ${{ secrets.GITHUB_TOKEN }}
       - name: "Show `git diff --stat`"
         run: git diff --stat gh-pages^ gh-pages || echo "(No diffs)"

.github/workflows/release.yml

@@ -146,5 +146,6 @@ jobs:
           .github/scripts/docs-build-deploy "${RELEASE_TAG_NAME}" latest --update-aliases --push
         env:
           RELEASE_TAG_NAME: ${{ github.event.release.tag_name }}
+          GH_TOKEN:  ${{ secrets.GITHUB_TOKEN }}
       - name: "Show `git diff --stat`"
         run: git diff --stat gh-pages^ gh-pages || echo "(No diffs)"