use sync.Once instead to maintain the const optimisations and avoid r…

…aces

internal/build/build.go

@@ -1,16 +1,29 @@
 // Package build contains information about they type of build currently running.
 package build
 
-import "fyne.io/fyne/v2"
+import (
+	"sync"
+
+	"fyne.io/fyne/v2"
+)
+
+var (
+	migrateCheck sync.Once
+
+	migratedFyneDo bool
+)
 
 func MigratedToFyneDo() bool {
 	if DisableThreadChecks {
 		return true
 	}
 
-	v, ok := fyne.CurrentApp().Metadata().Migrations["fyneDo"]
-	if ok {
-		return v
-	}
-	return false
+	migrateCheck.Do(func() {
+		v, ok := fyne.CurrentApp().Metadata().Migrations["fyneDo"]
+		if ok {
+			migratedFyneDo = v
+		}
+	})
+
+	return migratedFyneDo
 }