minor usability improvements

[tenuous-guidance]

A few small changes to my previous MR now I'm trying to use it:

• Cow so that Strings can be passed without having to hold their lifetime
• Clone is convenient for testing
• from_env does need need &mut self

I've not added a version bump here, as it doesn't look like 0.3.0 has been released yet.

[TheNeikos]

Looks good to me!

Thank you

bors merge

[bors[bot]]

Build succeeded:

• check envious-clippy [x86_64-linux]
• check envious-fmt [x86_64-linux]
• check envious [x86_64-linux]
• Evaluate flake.nix