GitHub event payload (`GITHUB_EVENT_PATH`) not mounted into devcontainer

[Sam13]

Relates to: https://github.com/devcontainers/ci/issues/188

GITHUB_EVENT_PATH is another GitHub action specific file which should be mounted into the devcontainer as already done for other files of this kind in https://github.com/devcontainers/ci/pull/219

See https://docs.github.com/en/actions/reference/workflows-and-actions/variables#default-environment-variables