nix: zstd-compressed debug sections in devShell

This cuts the size of a final debug binary in half (~250MiB -> 127MiB)
in my dev shell, and saves about 15% of total `target/` directory size
when a full build from scratch happens (2GiB -> 1.7GiB). Let's take an
easy free win.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

flake.nix

@@ -178,7 +178,7 @@
 
           export RUSTFLAGS="-Zthreads=0"
         '' + pkgs.lib.optionalString useMoldLinker ''
-          export RUSTFLAGS+=" -C link-arg=-fuse-ld=mold"
+          export RUSTFLAGS+=" -C link-arg=-fuse-ld=mold -C link-arg=-Wl,--compress-debug-sections=zstd"
         '' + darwinNextestHack;
       };
     }));