Restore `bazel mod deps` in post-submit config. (#2304)

This change is a partial revert of
2c23d7c47604a4984a20dcd19a66fb71f2ddba00 to fix breakages such as
https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/4676.

Progress towards
https://github.com/bazelbuild/continuous-integration/issues/2272.
3 files changed