chore: shard Cargo build cache by workspace

.cargo/config.toml

@@ -1,6 +1,3 @@
-# Keep intermediate build artifacts outside individual worktrees and reuse them
-# across parallel development of this repository. Final artifacts remain in each
-# worktree's target directory.
 [build]
-build-dir = "{cargo-cache-home}/build/yoi"
+build-dir = "{cargo-cache-home}/build/{workspace-path-hash}"
 incremental = false

devshell.nix

@@ -8,13 +8,16 @@ pkgs.mkShell {
     cargo
     pkgs.sccache
   ];
-  RUSTC_WRAPPER = "${pkgs.sccache}/bin/sccache";
+
   # sccache is additive to Cargo's shared build-dir, so keep its disk usage bounded.
+  RUSTC_WRAPPER = "${pkgs.sccache}/bin/sccache";
   SCCACHE_CACHE_SIZE = "5G";
+
   buildInputs = with pkgs; [
     pkg-config
     openssl
   ];
+
   shellHook = ''
     if repo_root="$(git rev-parse --show-toplevel 2>/dev/null)"; then
       : # export YOI_POD_RUNTIME_COMMAND="$repo_root/target/debug/yoi"