Fix hugo benchmark

Appending to the site 13 times doesn't sound realistic.

commands/benchmark.go

@@ -53,6 +53,7 @@ func benchmark(cmd *cobra.Command, args []string) error {
 			return err
 		}
 		for i := 0; i < benchmarkTimes; i++ {
+			mainSite = nil
 			_ = buildSite()
 		}
 		pprof.WriteHeapProfile(f)
@@ -71,6 +72,7 @@ func benchmark(cmd *cobra.Command, args []string) error {
 		pprof.StartCPUProfile(f)
 		defer pprof.StopCPUProfile()
 		for i := 0; i < benchmarkTimes; i++ {
+			mainSite = nil
 			_ = buildSite()
 		}
 	}