all: specify explicit JSON format for time.Duration (#17307)

The default representation of time.Duration has different
JSON representation between v1 and v2.

Apply an explicit format flag that uses the v1 representation
so that this behavior does not change if serialized with v2.

Updates tailscale/corp#791

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

net/speedtest/speedtest.go

@@ -24,7 +24,7 @@ const (
 // conduct the test.
 type config struct {
 	Version      int           `json:"version"`
-	TestDuration time.Duration `json:"time"`
+	TestDuration time.Duration `json:"time,format:nano"`
 	Direction    Direction     `json:"direction"`
 }