ci: remove git safe.directory workaround from container jobs

Problem

Four container jobs in .github/workflows/ci.yml run a manual workaround:

- name: fix git safe.directory
  run: git config --global --add safe.directory "$GITHUB_WORKSPACE"

This appears in validate, doccheck, test_skopeo, and ostree_rs_ext.

Why

Sandbox GHA migration review asked why this is needed when actions/checkout already configures safe.directory. mtrmac ACK'd removing it but it was never finished.

Draft PR #2885 started this work but remains [draft/DNM] and stale.

Related

• Draft PR #2885 — started but not merged
• Sandbox PR #1 review (mtrmac)