Specify core dimension for linux-code-coverage-generation

This should fix a NO_RESOURCE error due to a change of bot from 8 core
to 32 core.

TBR=nodir

Bug: 891884
Change-Id: I7e496871a1b531e50e95cb55b82193519f8977bf
Reviewed-on: https://chromium-review.googlesource.com/c/1260327Reviewed-by: Roberto Carrillo <robertocn@chromium.org>
Commit-Queue: Roberto Carrillo <robertocn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596528}

infra/config/global/cr-buildbucket.cfg

@@ -1819,6 +1819,7 @@ buckets {
     builders {
       name: "linux-code-coverage-generation"
       mixins: "code-coverage"
+      dimensions: "cores:32"
     }
 
     builders {