Restyle nix-shell segment

[jonascarpay]

This is a patch that I (and some friends) have been locally running, that I think makes the nix segment nicer to use.

Before:

After:

Having just the logo instead of the text should be fine because

• You rarely care about whether you're in a pure/impure shell
• If like most nix users you're in nix-shells a lot it's an annoyingly long segment

I also changed the color to more closely match the blue/white nix colors

[jonascarpay]

friendly ping, also, rebased

[fgaz]

This uses a character from the patched font even in compatible/flat modes. I think it should fall back to text in those cases, or better yet there could be an option to choose the style