cmd/go: make cfg.BuildContext.ToolTags same order with build.Default.ToolTags

So it's consistent when running "go list -f '{{context.ToolTags}}'" and
printing the content of "build.Default.ToolTags".

Updates #45454

Change-Id: I7a3cbf3cdf9a6ce2b8c89e9bcf5fc5e9086d48e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/422615
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>

Author: cuonglm

src/cmd/go/internal/cfg/cfg.go

@@ -237,9 +237,12 @@ func init() {
 	CleanGOEXPERIMENT = Experiment.String()
 
 	// Add build tags based on the experiments in effect.
-	for _, exp := range Experiment.Enabled() {
-		BuildContext.ToolTags = append(BuildContext.ToolTags, "goexperiment."+exp)
+	exps := Experiment.Enabled()
+	expTags := make([]string, 0, len(exps)+len(BuildContext.ToolTags))
+	for _, exp := range exps {
+		expTags = append(expTags, "goexperiment."+exp)
 	}
+	BuildContext.ToolTags = append(expTags, BuildContext.ToolTags...)
 }
 
 // An EnvVar is an environment variable Name=Value.