net/netmon: disable time jump monitoring on Plan 9

Updates #5794

Change-Id: I0f96383dea2ad017988d300df723ce906debb007
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netmon/netmon.go

@@ -596,7 +596,7 @@ func (m *Monitor) pollWallTime() {
 //
 // We don't do this on mobile platforms for battery reasons, and because these
 // platforms don't really sleep in the same way.
-const shouldMonitorTimeJump = runtime.GOOS != "android" && runtime.GOOS != "ios"
+const shouldMonitorTimeJump = runtime.GOOS != "android" && runtime.GOOS != "ios" && runtime.GOOS != "plan9"
 
 // checkWallTimeAdvanceLocked reports whether wall time jumped more than 150% of
 // pollWallTimeInterval, indicating we probably just came out of sleep. Once a