[infra] speed up {V8 Linux - arm - sim - debug}

The bot finishes close to our timeout SLO. Example failure:
https://ci.chromium.org/p/v8/builders/ci/V8%20Linux%20-%20arm%20-%20sim%20-%20debug/17582

TBR=machenbach@chromium.org
CC=​solanes@chromium.org,rmcilroy@chromium.org,mslekova@chromium.org

Bug: v8:9633
Change-Id: Ib855d06dce21c13119e38bba2455c5b7cc470160
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1760810
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Auto-Submit: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63256}

infra/testing/builders.pyl

@@ -1401,13 +1401,13 @@
         'name': 'mozilla',
         'suffix': 'armv8-a',
         'test_args': ['--extra-flags', '--enable-armv8'],
-        'shards': 2,
+        'shards': 3,
       },
       {
         'name': 'test262',
         'suffix': 'armv8-a',
         'test_args': ['--extra-flags', '--enable-armv8'],
-        'shards': 2,
+        'shards': 3,
       },
       {
         'name': 'v8testing',