feat: Default to PIXI_ENVIRONMENT_NAME in pixi list

[sumanth-manchala]

closes #893

I went ahead with @pavelzw suggestion, please let me know if this needs to thought through

[baszalmstra]

This also affects #889, there is a discussion going there about whether or not adding the environment variable is a good idea. I'd like to wait until that is resolved.

[sumanth-manchala]

While testing this, I found the below issue

EnvironmentName::Default != EnvironmentName::Named("default") when searching in IndexMap
https://github.com/prefix-dev/pixi/blob/main/src/project/manifest/mod.rs#L812

Any recommendation on tackling this?

[baszalmstra]

I think we should maybe just refactor EnvironmentName into

struct EnvironmentName(String)

if the name is "default" it is the default environment name.

[baszalmstra]

Alternatively you could first parse the String argument into EnvironmentName using FromStr or change the type of the env arg from String to EnvironmentName.

[ruben-arts]

This was fixed in #1080