Avoid accidentally initializing pyenv in CI images.

Don't initialize pyenv automatically in root login shells: don't copy the updated .bashrc from the intermediate build images to the final image. This can cause confusion about which Python installation can or will be detected by CMake in a CI job.

The primary use case for pyenv is to prepare Python venvs, which are activated explicitly when needed, and which (once created) do not require pyenv to be initialized in order to be activated.