runtime: update timers.len with Store instead of Add

runtime: update timers.len with Store instead of Add

Writes to timers.len are protected by the timers.lock.
There is no need to use an Add instead of a Store,
and the code is clearer (and perhaps slightly faster)
using the Store.

Change-Id: Icc6caef1b7405adec55c9b55b999b71de7d97484
Reviewed-on: https://go-review.googlesource.com/c/go/+/564976
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>