fix(ci): disable cgo for pure Go tracewriter tests

The tracewriter package includes rust_writer.go with cgo, which
requires libcodetracer_trace_writer_ffi. Set CGO_ENABLED=0 for
test-tracewriter target to exclude the Rust FFI code.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: zah

justfile

@@ -4,7 +4,7 @@ test:
 
 # Run tracewriter package tests only (no cgo required)
 test-tracewriter:
-  go test ./tracewriter/ -v -run 'TestGoWriter'
+  CGO_ENABLED=0 go test ./tracewriter/ -v -run 'TestGoWriter'
 
 # Run tracewriter tests including Rust FFI (requires cgo + FFI library)
 test-tracewriter-ffi: