Drop the Slack action checkout workaround from the latest release tests

[albertvillanova]

This PR removes the extra checkout that the daily latest-release test workflow needed to resolve the local Slack action.

Motivation

tests_latest.yml checks out a release branch instead of the branch running the workflow. When the local post-slack composite action was introduced, the release branch in use at the time (v1.10-release) predated it, so ./.github/actions/post-slack could not be resolved and the action had to be checked out separately.

That is no longer the case: the workflow now runs against v1.13-release, which ships the action, and its copy is identical to the one on main, so the inputs passed by the Slack step still match.

Solution

Resolve the action from the workspace, as every other workflow in the repository already does.

Changes

• Remove the Check out the Slack action step and its comment
• Point the Post to Slack step at ./.github/actions/post-slack

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Note

Low Risk
CI-only change to how the Slack notification action is resolved; no application or runtime behavior changes.

Overview
Simplifies the daily tests latest release workflow now that v1.13-release includes the local Slack composite action.

Removes the extra actions/checkout step that sparse-checked out .github/actions from the triggering branch, and points Post to Slack at ./.github/actions/post-slack like the rest of the repo’s workflows.

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