Command line args for juliainstaller

`juliainstaller` should have command line args for
- choosing a default channel
- values for all configuration options
- a `--force` or `--quiet` or `--yes` option that installs with all defaults and no user interaction.

We should then also surface those flags in the shell script.

Start by locating the juliainstaller command entry point and the shell script that invokes it. Review the existing configuration options and installer flow, then define command-line handling for the requested channel, configuration values, and non-interactive installation; done means the flags work consistently through the shell script.

Written by the indexing model from the issue text.