fix: override both rustc-unwrapped and rustc wrapper for symlink_file patch to propagate

flake.nix

@@ -97,13 +97,16 @@
         # Rustc bootstrap: symlink_file panics with "File exists" during
         # s390x cross-compilation. Multiple call sites use t!(symlink_file(...)).
         # Patch the symlink_file method body to remove existing dest first.
-        (final: prev: {
+        (final: prev: let
+          patchedRustcUnwrapped = prev.rustc-unwrapped.overrideAttrs (old: {
+            patches = (old.patches or [ ]) ++ [
+              ./patches/rustc-symlink-file-eexist.patch
+            ];
+          });
+        in {
+          rustc-unwrapped = patchedRustcUnwrapped;
           rustc = prev.rustc.override {
-            rustc-unwrapped = prev.rustc.unwrapped.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
-              '';
-            });
+            rustc-unwrapped = patchedRustcUnwrapped;
           };
         })
       ];

patches/rustc-symlink-file-eexist.patch

@@ -0,0 +1,11 @@
+--- a/src/bootstrap/src/lib.rs
++++ b/src/bootstrap/src/lib.rs
+@@ -1990,7 +1990,7 @@ impl Build {
+         use std::os::unix::fs::symlink as symlink_file;
+         #[cfg(windows)]
+         use std::os::windows::fs::symlink_file;
+-        if !self.config.dry_run() { symlink_file(src.as_ref(), link.as_ref()) } else { Ok(()) }
++        if !self.config.dry_run() { let _ = std::fs::remove_file(link.as_ref()); symlink_file(src.as_ref(), link.as_ref()) } else { Ok(()) }
+     }
+
+     /// Returns if config.ninja is enabled, and checks for ninja existence,