feat: export rustc, cargo, rustfmt, sccache for s390x

Adds rustc bootstrap patch to fix symlink_file "File exists" panic
during s390x cross-compilation (lib.rs:1823). Removes existing
destination before creating symlink.

flake.nix

@@ -94,6 +94,18 @@
             });
           };
         })
+        # Rustc bootstrap: symlink_file panics with "File exists" during
+        # s390x cross-compilation (lib.rs:1823 calls t!(self.symlink_file(link, dst))).
+        # Patch symlink_file in lib.rs to remove existing destination first.
+        (final: prev: {
+          rustc = prev.rustc.overrideAttrs (old: {
+            postPatch =
+              (old.postPatch or "")
+              + ''
+                sed -i 's|if !self.config.dry_run() { symlink_file(src.as_ref(), link.as_ref())|if !self.config.dry_run() { let _ = std::fs::remove_file(link.as_ref()); symlink_file(src.as_ref(), link.as_ref())|' src/bootstrap/src/lib.rs
+              '';
+          });
+        })
       ];
 
       # Native builds
@@ -149,7 +161,13 @@
             crossOnlyPackages =
               {
                 "s390x-linux" = {
-                  inherit (pkgs) attic-client;
+                  inherit (pkgs)
+                    attic-client
+                    rustc
+                    cargo
+                    rustfmt
+                    sccache
+                    ;
                   nix = pkgs.nixVersions.nix_2_28;
                 };
               }