ci(workflow): set GOPROXY for Go builds on GitHub Actions

- Use `GOPROXY=https://proxy.golang.org,direct` to speed up module downloads
- Mitigates network flakiness (e.g., checksum DB timeouts/rate limits)
- `,direct` provides fallback for private/unproxyable modules
- No build logic changes; only affects dependency resolution across all matrix targets

.github/workflows/build.yml

@@ -25,6 +25,8 @@ jobs:
           - android-arm64
     name: Build
     runs-on: ${{ matrix.platform }}
+    env:
+      GOPROXY: https://proxy.golang.org,direct
     steps:
 
       - name: Checkout