cmd/tailscaled: fix windows logtail integration

I broke it in 1.17.x sometime while rewiring some logs stuff,
mostly in 0653efb092 (but with a handful
of logs-related changes around that time)

Fixes tailscale/corp#3265

Change-Id: Icb5c07412dc6d55f1d9244c5d0b51dceca6a7e34
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
(cherry picked from commit 59f4f33f60)

cmd/tailscaled/tailscaled_windows.go

@@ -78,7 +78,10 @@ func (service *ipnService) Execute(args []string, r <-chan svc.ChangeRequest, ch
 		// Make a logger without a date prefix, as filelogger
 		// and logtail both already add their own. All we really want
 		// from the log package is the automatic newline.
-		logger := log.New(os.Stderr, "", 0)
+		// We start with log.Default().Writer(), which is the logtail
+		// writer that logpolicy already installed as the global
+		// output.
+		logger := log.New(log.Default().Writer(), "", 0)
 		ipnserver.BabysitProc(ctx, args, logger.Printf)
 	}()
 
@@ -114,6 +117,9 @@ func beWindowsSubprocess() bool {
 	}
 	logid := os.Args[2]
 
+	// Remove the date/time prefix; the logtail + file logggers add it.
+	log.SetFlags(0)
+
 	log.Printf("Program starting: v%v: %#v", version.Long, os.Args)
 	log.Printf("subproc mode: logid=%v", logid)