Reduce the calls to the python interpreter during initialization (#6312)

* Reduce the calls to the python interpreter during initialization

This should reduce the delay the users experience when sourcing the
setup file to activate shell support. It works by generating at once
all the commands that needs to evaluated (they are stored in
a string and later `eval`ed by the shell).

* setup_env.sh: changed `read` with an equivalent magic

For some reason `read` breaks when sourced from a running script.
Change the incantation we use to construct the unique python command
that will be evaluated.

* setup_env.sh: python command now constructed with `printf` for portability

This recovers the support for `zsh` that was broken in previous commits.

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