cmd/tailscale/cli: fix time in tailscale routecheck (#19956)

When running `tailscale netcheck`, the reported timestamp used to be
in UTC and formatted according to RFC 3339 with a `T` to separate the
date from the time:

	sfllaw@h2co3:~$ tailscale netcheck | head -n3

	Report:
		* Time: 2026-06-01T21:12:32.252620138Z

This is machine-readable time leaking out to the user interface. Times
in normal commands are formatted for humans to read:

	sfllaw@h2co3:~$ date
	Mon 01 Jun 2026 02:39:14 PM PDT
	sfllaw@h2co3:~$ journalctl -t tailscaled | tail -n1
	Jun 01 14:35:21 h2co3 tailscaled[3328921]: wgengine: sending TSMP disco key advertisement to 100.90.144.102
	sfllaw@h2co3:~$ timedatectl show
	Timezone=America/Los_Angeles
	LocalRTC=no
	CanNTP=yes
	NTP=yes
	NTPSynchronized=yes
	TimeUSec=Mon 2026-06-01 14:38:32 PDT
	RTCTimeUSec=Mon 2026-06-01 14:38:32 PDT
	sfllaw@h2co3:~$ uptime --since
	2026-05-15 07:37:45

This PR makes the times printed by the CLI commands consistent:

- For `tailscale routecheck`, it now prints local time as
  `2026-05-15 07:37:45-07:00`.
- For `netlogfmt`, it has always printed local time with a space,
  but now includes the time zone.
- All machine-readable outputs continue to be standard RFC 3339 in
  UTC, i.e. `--format=json`.

As part of a general cleanup, this PR also adds standard common
time.Format layouts as tstime constants.

Fixes #19928

Signed-off-by: Simon Law <sfllaw@tailscale.com>

log/filelogger/log.go

@@ -16,6 +16,7 @@ import (
 	"sync"
 	"time"
 
+	"tailscale.com/tstime"
 	"tailscale.com/types/logger"
 )
 
@@ -106,13 +107,8 @@ func (w *logFileWriter) appendToFileLocked(out []byte) {
 	}
 	out = removeDatePrefix(out)
 	if w.f != nil {
-		// RFC3339Nano but with a fixed number (3) of nanosecond digits:
-		const formatPre = "2006-01-02T15:04:05"
-		const formatPost = "Z07:00"
-		fmt.Fprintf(w.f, "%s.%03d%s: %s",
-			now.Format(formatPre),
-			now.Nanosecond()/int(time.Millisecond/time.Nanosecond),
-			now.Format(formatPost),
+		fmt.Fprintf(w.f, "%s: %s",
+			now.Format(tstime.DateTTimeMilliZ),
 			out)
 	}
 }