[gn] Fix sysroot usage with gcc

BUG=chromium:474921
TBR=jochen@chromium.org, vogelheim@chromium.org,
NOTRY=true

Review-Url: https://codereview.chromium.org/2124003003
Cr-Commit-Position: refs/heads/master@{#37575}

infra/mb/mb_config.pyl

@@ -425,7 +425,7 @@
     },
 
     'gcc': {
-      'gn_args': 'is_clang=false',
+      'gn_args': 'is_clang=false use_sysroot=false',
       'gyp_defines': 'clang=0',
     },