rm regex

[estk]

static RE: once_cell::sync::Lazy<regex::Regex> =
    once_cell::sync::Lazy::new(|| regex::Regex::new(r#"\$ENV\{([\w][\w|\d|\.|_]*)\}"#).unwrap());

Didn't this get changed in log4rs? Or rather, wasn't the regex stuff removed? If so maybe we should consider removing it here as well, as a part of future work.