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This PR moves our repo setup to onCreateCommand from postCreateCommand in order to resolve a race condition between conda init and the default shell in VS Code. You can test this new devcontainer.json by creating a codespace for my fork/branch: https://github.com/codespaces/new?hide_repo_select=true&ref=jungaretti%2Fupdate-devcontainer&repo=576410652

postCreateCommand doesn't block interaction with your codespace. You can use other terminals and move around editors while it runs. On the other hand, onCreateCommand runs before you interact with your codespace. You won't be able to open a terminal until it finishes. onCreateCommand is a better choice for running our setup script because we need to run conda init before using the terminal.