fix: avoid race between replicas on start (#12344)

DERP mesh key setup would do a SELECT and then an INSERT on failure, without a lock. During some testing with multiple replicas, I managed to cause a replica to crash due to them initializing simultaneously.

Fixes:

Encountered an error running "coder server"
create coder API: insert mesh key: pq: duplicate key value violates unique constraint "site_configs_key_key"

Co-authored-by: Cian Johnston <cian@coder.com>

coderd/database/dbauthz/dbauthz_test.go

@@ -1874,6 +1874,7 @@ func (s *MethodTestSuite) TestSystemFunctions() {
 		check.Args(u.ID).Asserts(rbac.ResourceSystem, rbac.ActionRead)
 	}))
 	s.Run("GetDERPMeshKey", s.Subtest(func(db database.Store, check *expects) {
+		db.InsertDERPMeshKey(context.Background(), "testing")
 		check.Args().Asserts(rbac.ResourceSystem, rbac.ActionRead)
 	}))
 	s.Run("InsertDERPMeshKey", s.Subtest(func(db database.Store, check *expects) {