Add win-pixel-builder-rel and win-pixel-tester-rel to buildbucket.

Currently test failed with: Buildbucket returned error "BUILDER_NOT_FOUND":
 builder "win-pixel-builder-rel" not found in bucket "chromium/ci"
https://luci-scheduler.appspot.com/jobs/chromium/win-pixel-builder-rel/9077335688480808992
This cl should fix the error.

Bug: 965769
Change-Id: If22152956afb8a518032a225fa6ef8183320f329
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1639643Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Sven Zheng <svenzheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665607}

infra/config/cr-buildbucket.cfg

@@ -1809,6 +1809,14 @@ buckets {
       name: "win-jumbo-rel"
       mixins: "win-ci"
     }
+    builders {
+      name: "win-pixel-builder-rel"
+      mixins: "win-ci"
+    }
+    builders {
+      name: "win-pixel-tester-rel"
+      mixins: "win-ci"
+    }
     builders {
       name: "win32-arm64-rel"
       dimensions: "os:Windows-10"