[release-branch.go1.13] cmd/go/internal/modget: synchronize writes to modOnly map in runGet

Adds an additional lock around an access to modOnly.

Updates #35317
Fixes #35318

Change-Id: Ia1e75f9a674ec2a2c0489b41283c1cd3e7924d1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/209237


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
(cherry picked from commit 9174e2c0)
Reviewed-on: https://go-review.googlesource.com/c/go/+/209222
Run-TryBot: Alexander Rakoczy <alex@golang.org>