fixes race condition Ecto.Repo.Supervisor if adapter needs compiling

[evadne]

• Follow-up of Ecto.Repo.Supervisor.compile_config/2 should use ensure_compiled? instead of ensure_loaded? #2822

• Previously, Ecto.Repo.Supervisor uses Core.ensure_loaded?/1 which could cause errors when the adapter is not yet compiled.

• See https://hexdocs.pm/elixir/Code.html#ensure_loaded/1-code-loading-on-the-erlang-vm

  When invoked, ensure_compiled/1 halts the compilation of the caller until the module given to ensure_compiled/1 becomes available or all files for the current project have been compiled. If compilation finishes and the module is not available, an error tuple is returned.

[josevalim]

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