build: specify platforms explicitly (to workaround regression in Turbo Bob)

Author: joonas-fi

.config/turbobob.json

@@ -5,9 +5,7 @@
 	"docker_images": [
 		{
 			"image": "fn61/buildkit-golang",
-			"dockerfile_path": "Dockerfile",
-			"tag_latest": true,
-			"auth_type": "creds_from_env"
+			"tag_latest": true
 		}
 	]
 }

.github/workflows/build.yml

@@ -8,6 +8,8 @@ jobs:
     steps:
     - uses: actions/checkout@v4
 
+    - uses: docker/setup-buildx-action@v3
+
     - name: Build
       run: |
         set -eu