log/slog: fix window race builder

Bench log file is created non-portably, only works on system where
"/tmp" existed and "/" is path separator.

Fixing this by using portable methods from std lib.

Updates golang#56345

Change-Id: I1f6b6b97b913ca56a6053beca7025652618ecbf3
Reviewed-on: https://go-review.googlesource.com/c/go/+/478355
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: cuonglm

src/log/slog/json_handler_test.go

@@ -14,6 +14,7 @@ import (
 	"log/slog/internal/buffer"
 	"math"
 	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 	"time"
@@ -201,7 +202,7 @@ func BenchmarkPreformatting(b *testing.B) {
 		}),
 	}
 
-	outFile, err := os.Create("/tmp/bench.log")
+	outFile, err := os.Create(filepath.Join(b.TempDir(), "bench.log"))
 	if err != nil {
 		b.Fatal(err)
 	}