go/build/constraint: use strings.Builder instead of for { str+=str }

(This works around a bug in the stringsbuilder modernizer.)

For #76476

Change-Id: I1cb8715fd79c0363cb9c159686eaeb3482c93228
Reviewed-on: https://go-review.googlesource.com/c/go/+/724721
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Alan Donovan <adonovan@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: adonovan

src/go/build/constraint/expr.go

@@ -515,18 +515,18 @@ func PlusBuildLines(x Expr) ([]string, error) {
 	// Prepare the +build lines.
 	var lines []string
 	for _, or := range split {
-		line := "// +build"
+		var line strings.Builder
+		line.WriteString("// +build")
 		for _, and := range or {
-			clause := ""
+			line.WriteString(" ")
 			for i, lit := range and {
 				if i > 0 {
-					clause += ","
+					line.WriteString(",")
 				}
-				clause += lit.String()
+				line.WriteString(lit.String())
 			}
-			line += " " + clause
 		}
-		lines = append(lines, line)
+		lines = append(lines, line.String())
 	}
 
 	return lines, nil