[infra/config] set ssd:1 dimension for clang ToT Mac builders

This is fix for
https://chromium-review.googlesource.com/c/chromium/src/+/1654757

Bug: 956776
Change-Id: I700924f1962ce20110beb5e37ecb02ea3fa616ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1659661Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669130}

infra/config/cr-buildbucket.cfg

@@ -429,6 +429,7 @@ builder_mixins {
   mixins: "clang-ci"
   mixins: "mac"
   dimensions: "cores:24"
+  dimensions: "ssd:1"
 }
 
 builder_mixins {