fish: conf.d/setup: Set environment variable for Poetry

If and when we get to using Poetry, make sure it creates
and uses the virtual environment inside the project root.

https://python-poetry.org/docs/configuration/#virtualenvsin-project

fish/.config/fish/conf.d/setup.fish

@@ -51,6 +51,9 @@ function __fish_setup_on_tty_login --description "Set up environment on tty logi
     # Ripgrep requires this environment variable for knowing the config file path
     set -gx RIPGREP_CONFIG_PATH {$HOME}/.config/ripgrep/ripgreprc
 
+    # Make Poetry create virtualenv inside the project root
+    set -gx POETRY_VIRTUALENVS_IN_PROJECT true
+
     # Required for all environment variables to be set correctly, especially
     # in tmux session service.
     dbus-update-activation-environment --systemd --all