fix: run cargo install as root in stylus.Dockerfile

The cargo install was failing with permission denied because it ran
as the agent user but the cargo registry had root-owned files.
Move cargo install to run as root (like other Dockerfiles do).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: drewstone

infra/stylus.Dockerfile

@@ -1,10 +1,10 @@
 FROM foundry:latest
 
 USER root
-RUN rustup target add wasm32-unknown-unknown
+RUN rustup target add wasm32-unknown-unknown && \
+    cargo install cargo-stylus && \
+    chmod -R a+w /usr/local/cargo
 
 USER agent
 
-RUN cargo install cargo-stylus
-
 LABEL description="stylus infrastructure layer"