Allow environment variables definition on activation without setting them in pixi.toml

[yonil7]

Problem description

1. [activation.env] section does not support some kind of composite env var syntax that will allow defining env variables such as: PGDATA="${PIXI_PROJECT_ROOT}/my_pg_data_folder"
2. [activation.script] section only allows running scripts files. (so I cant do: export PGDATA="${PIXI_PROJECT_ROOT}/my_pg_data_folder or source my_env_vars_definition_script.sh there)
   This means there is no way to define "composite" env vars on pixi activation. (not in pixi.toml syntax and not by running something else on activation)

[ruben-arts]

That would indeed be nicer. Currently the activation env is thrown on the pile of env vars that pixi adds to the activation.

What does work in this case is:
pixi.toml:

[activation]
scripts = ["activation.sh"]

activation.sh:

export PGDATA="$PIXI_PROJECT_ROOT/my_pg_data_folder"

# Make sure you escape the `$` on your shell
pixi run "echo \$PGDATA"