rust: add includable CQ builder for testing

This adds a new CQ builder (default disabled, i.e. includable_only) to
test the new Rust recipe steps added in crrev.com/c/6774006 for 64-bit
linux.

Change-Id: I7ebb89041b40b8b2cde4746259b6ffe5455cb027
Bug: 42290446
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/80567
Auto-Submit: Lily Chen <chlily@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

infra/config/generated/commit-queue.cfg

@@ -166,6 +166,10 @@ config_groups {
       builders {
         name: "boringssl/try/linux_rel"
       }
+      builders {
+        name: "boringssl/try/linux_rust"
+        includable_only: true
+      }
       builders {
         name: "boringssl/try/linux_sde"
         includable_only: true