[build] Disable sysroot on gcov coverage builder

The sysroot only contains headers for gcc6, not gcc4.8 which is the version installed on the coverage bots.

Bug: chromium:837238
Change-Id: Ifd9b53dae510b969b4ef5ec8c1c2684eb92a94fa
Reviewed-on: https://chromium-review.googlesource.com/1059150Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53186}

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', 'no_custom_libcxx'],
+      'release_bot', 'x64', 'coverage', 'gcc', 'no_custom_libcxx', 'no_sysroot'],
     'release_x64_internal': [
       'release_bot', 'x64', 'v8_enable_embedded_builtins',
       'v8_snapshot_internal'],