Install Pixi from R, with Pixi's official installer, so you don't need a
terminal. It installs Pixi into ~/.pixi/bin (or $PIXI_HOME/bin), where
rpix finds it straight away. When an rpix function can't find Pixi, it
offers to run install_pixi().
By default, the installer also adds Pixi to your shell's PATH, so pixi
works in a terminal too, after you open a new one. If Pixi is installed
already, nothing happens unless force = TRUE; update an existing Pixi with
pixi self-update in a terminal.
The installer is Pixi's own, install.sh (or install.ps1 on Windows)
from pixi.sh. It may send prefix.dev an anonymous install count; set
PIXI_NO_TELEMETRY=1 to turn that off. See
https://pixi.prefix.dev/latest/installation/.