Fix Shell Activation For ZSH

This PR resolves an issue wherein poetry will clear the PATH on zsh. The activation script contains a number of if statements of the following form:

    # ...
    if ! [ -z "${_OLD_VIRTUAL_PATH:+_}" ] ; then
        PATH="$_OLD_VIRTUAL_PATH"
        export PATH
        unset _OLD_VIRTUAL_PATH
    fi

These constructs are valid in bash, although zsh fails to negate the conditional except when placed in emulation mode:

# Causes PATH to be cleared, leading to other issues
. /home/alecto/.cache/pypoetry/virtualenvs/poetry-E-aAiPjf-py3.10/bin/activate

When bash is used as an emulator to source the script, it functions correctly:

# Functions correctly
emulate bash -c '. /home/alecto/.cache/pypoetry/virtualenvs/poetry-E-aAiPjf-py3.10/bin/activate'

It's important to note that this doesn't place the entire shell into bash emulation mode; only the activate script is sourced as bash.

Any guidance on what tests should be added for this PR would be appreciated.