fix(engine/buildx): recipe used reserved 'default' builder name

'docker buildx create --name default' errors with 'default is a
reserved name and cannot be used to identify builder instance'.
The bundled builder always exists under that name; the recipe
should switch to it (buildx use default), not try to recreate it.

For the count==0 driver-rebuild branch, the new builder needs a
non-reserved name — using 'decnet-builder' as the example.

tests/fleet/test_deployer.py

@@ -176,7 +176,7 @@ class TestComposeWithRetry:
         )
         with pytest.raises(subprocess.CalledProcessError) as ei:
             deployer._compose_with_retry("up", "--build")
-        assert "buildx create --use" in ei.value.stderr
+        assert "buildx create --name decnet-builder" in ei.value.stderr
         assert "umount" not in ei.value.stderr
         assert "No leaked mounts (count=0)" in ei.value.stderr