fix: serialize rust cache uploads

README.md

@@ -20,7 +20,7 @@ jobs:
         with:
           toolchain: stable      # optional, defaults to stable
           cache-prefix: rust     # optional, defaults to rust
-          cache-version: v2      # optional, defaults to v2
+          cache-version: v3      # optional, defaults to v3
 
       - run: cargo build --release
       - run: cargo test
@@ -33,6 +33,10 @@ jobs:
 3. **Rust install** — Runs `rustup` with the minimal profile to install or verify the requested toolchain.
 4. **PATH setup** — Appends `$HOME/.cargo/bin` to `$GITHUB_PATH` so cargo/rustc are available in subsequent steps.
 
+The cache upload is serialized with `CACHE_UPLOAD_CONCURRENCY=1` because Gitea
+act_runner's cache server stores chunk metadata in BoltDB and can return 500s
+under large parallel uploads.
+
 ### Cache key format
 
 ```