fix(docker): preserve workspace Rust flags

Dockerfile

@@ -96,10 +96,12 @@ RUN --mount=type=cache,target=/usr/local/cargo/registry \
     if [ -n "$SYNCTV_BUILD_FEATURES" ]; then \
         build_flags="$build_flags --features $SYNCTV_BUILD_FEATURES"; \
     fi; \
-    RUSTFLAGS="-Clink-arg=-fuse-ld=lld -Clink-arg=-Wl,-z,pack-relative-relocs" \
     cargo \
-        build $build_flags \
-        --bin synctv && \
+        rustc $build_flags \
+        --bin synctv \
+        -- \
+        -Clink-arg=-fuse-ld=lld \
+        -Clink-arg=-Wl,-z,pack-relative-relocs && \
     cp "target/$target_profile_dir/synctv" /synctv
 
 # Stage 2: Runtime image