fix(ci): remove redundant rustup install now that base image already has it

The official rust:alpine base (unlike whatever image the old defunct
registry.nxtgauge.com mirror actually held) already ships a full
rustup-installed toolchain, so the extra `curl rustup.rs | sh` step
now fails with "cannot install while Rust is installed". Drop it and
the /root/.cargo/bin PATH override (the base image already sets PATH
to its own /usr/local/cargo/bin) - just add the musl target, which the
existing rustup binary can do directly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Dockerfile.simple

@@ -6,11 +6,11 @@ ARG SERVICE_NAME
 FROM ci.nxtgauge.com/admin/rust:alpine AS builder
 ARG SERVICE_NAME
 
-# Install build deps + rust toolchain (Alpine-packaged Rust lacks proc-macro support)
+# The official rust:alpine base already provides a full rustup-installed
+# toolchain (unlike Alpine's apk-packaged rust, which lacks proc-macro
+# support) - just add build deps and the musl target.
 RUN apk add --no-cache curl ca-certificates bash build-base musl-dev pkgconfig openssl-dev openssl-libs-static
 RUN update-ca-certificates
-RUN curl -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal --default-toolchain stable
-ENV PATH="/root/.cargo/bin:${PATH}"
 RUN rustup target add x86_64-unknown-linux-musl
 
 WORKDIR /app