[code coverage] Set coverage input properties for linux-chromeos-rel

This CL sets coverage input properties on linux-chromeos-rel so that
it will start instrumenting and processing code coverage data.

Bug: 1000367
Change-Id: I9335cd0512e95d2d0f1ec2b6caa69c62380b42d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1825734Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700439}

infra/config/cr-buildbucket.cfg

@@ -4355,6 +4355,7 @@ buckets {
       mixins: "chromeos-try"
       mixins: "goma-j150"
       mixins: "builderless"
+      mixins: "clang-coverage"
       name: "linux-chromeos-rel"
     }
     # This builder replicates linux-chromeos-rel for code coverage experiment.