Replace unmaintained directories crate

Fixes RUSTSEC-2020-0054 warning from cargo-audit/cargo-deny, follows the recommendation to switch to the new maintained `directories-next` crate fork

Only affects the cache directory determination for the environment and was a simple search'n'replace to this fork so don't think behavior has changed.

https://rustsec.org/advisories/RUSTSEC-2020-0054

crates/cache/src/config.rs

@@ -2,7 +2,7 @@
 
 use super::Worker;
 use anyhow::{anyhow, bail, Context, Result};
-use directories::ProjectDirs;
+use directories_next::ProjectDirs;
 use log::{trace, warn};
 use serde::{
     de::{self, Deserializer},