seth/dapp --use: look for systemwide solc installs

[d-xo]

Description

seth --use and dapp --use will now find copies of solc that have been installed systemwide from solc-versions or solc-static-versions on nixos systems, as well as the imperitive versions installed via nix-env.

Checklist

• tested locally
• added automated tests
• updated the docs
• updated the changelog

[MrChico]

hmmm, I thought the current way of doing this was meant to do this already? And also in a way that allowed non nixos users to find installed solc versions...

[d-xo]

The current way just finds stuff installed via nix-env, but not installed via configuration.nix.

I think this new way should work for both nix-env and systemwide installs anywhere that has nix, but I don't have systems like that to test on :/