Replace `strtobool` by a simpler check in `shell` command (#6191)

* test(commands): add tests for `shell`

* refactor(commands/shell): move venv check to dedicated function

* feat(commands/shell): replace `strtobool` with simpler check

Co-authored-by: Xinyuan Chen <45612704+tddschn@users.noreply.github.com>

* test(commands/shell): add tests for `_is_venv_activated`

* chore(cirrus): explicitly set `SHELL` env var

Co-authored-by: Xinyuan Chen <45612704+tddschn@users.noreply.github.com>
Co-authored-by: Bartosz Sokorski <b.sokorski@gmail.com>