test(selfupdate): make executable-bit assertion Unix-only

Windows file modes don't carry a Unix executable bit, so the
TestDownloadVerifyExtract perm check failed on windows-latest CI. The runner
only ever self-updates on systemd-Linux; guard the assertion with
runtime.GOOS so the (still meaningful) download/verify/extract coverage runs
on every platform while the Unix-only bit check is skipped on Windows.

Author: ysyneu

selfupdate/update_test.go

@@ -11,6 +11,7 @@ import (
 	"net/http/httptest"
 	"os"
 	"path/filepath"
+	"runtime"
 	"testing"
 
 	"github.com/flashcatcloud/flashduty-runner/protocol"
@@ -71,9 +72,13 @@ func TestDownloadVerifyExtract(t *testing.T) {
 	if string(got) != string(binBytes) {
 		t.Fatal("extracted bytes mismatch")
 	}
-	fi, _ := os.Stat(dest)
-	if fi.Mode().Perm()&0o100 == 0 {
-		t.Fatal("expected executable bit")
+	// Windows file modes don't carry a Unix executable bit, and the runner only
+	// ever self-updates on systemd-Linux — so this is a Unix-only assertion.
+	if runtime.GOOS != "windows" {
+		fi, _ := os.Stat(dest)
+		if fi.Mode().Perm()&0o100 == 0 {
+			t.Fatal("expected executable bit")
+		}
 	}
 
 	// bad checksum