fix: show global config in info command

[ruben-arts]

fixes #1013

[nichmor]

some findings:

• https://pixi.sh/latest/reference/pixi_configuration/
  in docs we said that
  To find the locations where pixi looks for configuration files, run pixi with -v or --verbose.

  Running cargo run -- info --verbose I can't find any logs about where it looks for global configuration.
  

• Running with -vvv I have following output:

DEBUG pixi_config: Loading config from /etc/pixi/config.toml
DEBUG pixi_config: Failed to load system config: /etc/pixi/config.toml (error: failed to read config from '/etc/pixi/config.toml')
DEBUG pixi_config: Loading config from /Users/graf/.config/pixi/config.toml
DEBUG pixi_config: Failed to load global config: /Users/graf/.config/pixi/config.toml (error: failed to read config from '/Users/graf/.config/pixi/config.toml')
DEBUG pixi_config: Loading config from /Users/graf/Library/Application Support/pixi/config.toml
DEBUG pixi_config: Failed to load global config: /Users/graf/Library/Application Support/pixi/config.toml (error: failed to read config from '/Users/graf/Library/Application Support/pixi/config.toml')
DEBUG pixi_config: Loading config from /Users/graf/.pixi/config.toml
DEBUG pixi_config: Failed to load global config: /Users/graf/.pixi/config.toml (error: failed to read config from '/Users/graf/.pixi/config.toml')
DEBUG pixi_config: Loading config from /Users/graf/projects/oss/pixi-3/.pixi/config.toml
DEBUG pixi_config: Failed to load local config: /Users/graf/projects/oss/pixi-3/.pixi/config.toml (error: failed to read config from '/Users/graf/projects/oss/pixi-3/.pixi

maybe we can change the debug message and just tell that config.toml is totally missing?

• If I don't have a config location,
  instead of this:
  Config locations:

maybe we can output something like this:

Config locations: no config file has been found

Other than that, it looks very good!