ci: fix Bad substitution in docs publish step

The gitea-pages push step used ${GITHUB_SHA::8}, a bash-only substring
expansion. The Gitea runner executes run: blocks with /bin/sh (dash),
which rejects it with "Bad substitution" and exits 2, failing the job
after the site had already built successfully.

Use cut(1) to shorten the SHA instead, which is POSIX sh compatible.

.gitea/workflows/publish-docs.yml

@@ -117,6 +117,8 @@ jobs:
           git config user.email "actions@gitea.tourolle.paris"
           git checkout -q -b gitea-pages
           git add -A
-          git commit -q -m "docs: publish site from ${GITHUB_SHA::8}"
+          # POSIX sh has no ${VAR::N} substring expansion — cut instead.
+          SHORT_SHA="$(printf '%s' "$GITHUB_SHA" | cut -c1-8)"
+          git commit -q -m "docs: publish site from ${SHORT_SHA}"
           echo "🚀 Force-pushing to gitea-pages"
           git push -f "$REMOTE" gitea-pages