pmb.config.chroot_home_symlinks: cache rust dirs (!1850)

Rust packaging is new and still a bit weird in Alpine and postmarketOS.
As of writing, we only have one package (squeekboard), and use cargo to
download the source of all dependencies at build time (several git
repositories!) and compile it. Usually, this is a no-go, but at least
until this is resolved properly, let's cache the downloads as suggested
in: https://doc.rust-lang.org/cargo/guide/cargo-home.html

Related: #1861

pmb/helpers/frontend.py

@@ -375,7 +375,8 @@ def zap(args):
     pmb.chroot.zap(args, dry=args.dry, http=args.http,
                    distfiles=args.distfiles, pkgs_local=args.pkgs_local,
                    pkgs_local_mismatch=args.pkgs_local_mismatch,
-                   pkgs_online_mismatch=args.pkgs_online_mismatch)
+                   pkgs_online_mismatch=args.pkgs_online_mismatch,
+                   rust=args.rust)
 
     # Don't write the "Done" message
     pmb.helpers.logging.disable()