Fix sccache coflict

devenv-rust.nix

@@ -26,6 +26,14 @@
     vulkan-tools
   ];
   env.RUSTC_WRAPPER = "sccache";
+  # Per-user sccache server socket. On the default shared TCP port (4226)
+  # the first user's server performs every rustc write for every user, so a
+  # second user's builds die with EACCES in their own target/ dir.
+  enterShell = ''
+    sccache_dir="''${XDG_RUNTIME_DIR:-/tmp/sccache-$(id -u)}"
+    mkdir -p "$sccache_dir"
+    export SCCACHE_SERVER_UDS="$sccache_dir/sccache.sock"
+  '';
   # https://devenv.sh/languages/
   languages.rust = {
     enable = true;