[build] Disable custom libcxx on gcov coverage builder

NOTRY=true
TBR=sergiyb@chromium.org

Bug: chromium:837238
Change-Id: Ib10cb178b707eace50d77d20a732a106904167fb
Reviewed-on: https://chromium-review.googlesource.com/1058790Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53172}

infra/mb/mb_config.pyl

@@ -415,7 +415,7 @@
     'release_x64_fuchsia_trybot': [
       'release_trybot', 'x64', 'fuchsia'],
     'release_x64_gcc_coverage': [
-      'release_bot', 'x64', 'coverage', 'gcc'],
+      'release_bot', 'x64', 'coverage', 'gcc', 'no_custom_libcxx'],
     'release_x64_internal': [
       'release_bot', 'x64', 'v8_enable_embedded_builtins',
       'v8_snapshot_internal'],
@@ -665,6 +665,10 @@
       'gn_args': 'is_clang=false',
     },
 
+    'no_custom_libcxx': {
+      'gn_args': 'use_custom_libcxx=false',
+    },
+
     'no_sysroot': {
       'gn_args': 'use_sysroot=false',
     },