test(update): make replace-executable test host-independent

Author: hjun1052

internal/workflow/update_test.go

@@ -357,6 +357,8 @@ func TestExtractBinaryHelpersAndReplaceCurrentExecutable(t *testing.T) {
 		restore := mockUpdateEnvironment(t)
 		defer restore()
 
+		updateRuntimeGOOS = "linux"
+		updateRuntimeGOARCH = "amd64"
 		target := filepath.Join(t.TempDir(), "xgit")
 		if err := os.WriteFile(target, []byte("old"), 0o755); err != nil {
 			t.Fatalf("write old binary: %v", err)