Fast fix of cores dimension to bring Linux Builder online.

It has been offline for 24 hours until machenbach@ pinged me
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20Builder

R=machenbach@chromium.org, nodir@chromium.org

No-Try: True
Change-Id: I29096b6a932dd215ed34600674c4785df83a338f
Reviewed-on: https://chromium-review.googlesource.com/975925Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545103}

infra/config/global/cr-buildbucket.cfg

@@ -473,6 +473,7 @@ buckets {
     builders {
       name: "Linux Builder"
       mixins: "linux-ci"
+      dimensions: "cores:32"
     }
 
     builders {