chore(ci): ensure we publish latest_snapshot container image (#9381)

We need to publish this tag on commits to `main`.

## Checklist

- [x] Change(s) are motivated and described in the PR description
- [x] Testing strategy is described if automated tests are not included
in the PR
- [x] Risks are described (performance impact, potential for breakage,
maintainability)
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed or label `changelog/no-changelog` is set
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/))
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))
- [x] If this PR changes the public interface, I've notified
`@DataDog/apm-tees`.

## Reviewer Checklist

- [ ] Title is accurate
- [ ] All changes are related to the pull request's stated goal
- [ ] Description motivates each change
- [ ] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- [ ] Testing strategy adequately addresses listed risks
- [ ] Change is maintainable (easy to change, telemetry, documentation)
- [ ] Release note makes sense to a user of the library
- [ ] Author has acknowledged and discussed the performance implications
of this PR as reported in the benchmarks PR comment
- [ ] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

.github/workflows/lib-injection.yml

@@ -18,6 +18,15 @@ jobs:
       ddtrace-version: v2.6.3
       image-tag: ${{ github.sha }}
 
+  build-and-publish-latest-snapshot-image:
+    if: github.ref == 'refs/heads/main'
+    uses: ./.github/workflows/lib-inject-publish.yml
+    secrets:
+      token: ${{ secrets.GITHUB_TOKEN }}
+    with:
+      ddtrace-version: v2.6.3
+      image-tag: latest_snapshot
+
   test-runner-test:
     needs:
       - build-and-publish-test-image