configureGitAuth() removed the actions/checkout auth header with
`git config --unset-all http.<server>/.extraheader`, which only edits the
repo-local config. Since actions/checkout v6.0.0 (backported to v5.0.1 and
v4.3.1) the header is written to a separate file under RUNNER_TEMP and
pulled in via include.path, so --unset-all on the local config is a no-op:
the code logged "No existing authentication headers to remove" while the
checkout credential (usually the workflow GITHUB_TOKEN) stayed usable by
git for the rest of the job.
Also clear the header from every included file, so it is neutralized under
both the pre-v6 (local) and v6+ (include) layouts. Includes that do not
define the header are left untouched.
Fixes#1510
