infra: linux-goma-rbe-staging-rel experiment 10% -> 20%

The bots occasionally reach 100% usage but most of the time are far
below 100%. https://screenshot.googleplex.com/63y7ueSWyay.png

We can increase to 20% and see the effect.

BUG=855319

Change-Id: I0c2c234d8ef04dc29afce514a39f26856ab20fe2
Reviewed-on: https://chromium-review.googlesource.com/c/1337219Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: smut <smut@google.com>
Commit-Queue: Simon Que <sque@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608586}

infra/config/branch/cq.cfg

@@ -118,7 +118,7 @@ verifiers {
       # https://crbug.com/855319
       builders {
         name: "linux-goma-rbe-staging-rel"
-        experiment_percentage: 10
+        experiment_percentage: 20
       }
       builders { name: "linux-jumbo-rel" }
       builders {