Lacros: Enable lacros-amd64-generic-rel on branches

This builder was only enabled on trunk due to lack of testing devices,
later, the lab team deployed ~25 Eve devices for us, but we never got
back to enable the builder on branches, and this CL fixes the problem.

Based on the capacity graph for builders and testers:
http://shortn/_Ed05hfQEeM
http://shortn/_PVSLQWbEj5

There is abundant capacity to absorb this ~5% load increase.

Additionally, this builder is extremely light as it only runs a few
small tests, and its P50 is at ~14min, so this is no risk of being the
bottleneck so as to regress CQ cycle time.

Change-Id: Ic2805af9db2d4b3c71358a20149e93d0abd44db8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3295593
Reviewed-by: Sven Zheng <svenzheng@chromium.org>
Reviewed-by: Gary Tong <gatong@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#944853}

infra/config/subprojects/chromium/ci.star

@@ -2086,6 +2086,7 @@ ci.chromiumos_builder(
 
 ci.chromiumos_builder(
     name = "lacros-amd64-generic-rel",
+    branch_selector = branches.STANDARD_MILESTONE,
     console_view_entry = consoles.console_view_entry(
         category = "lacros|x64",
         short_name = "rel",

infra/config/subprojects/chromium/try.star

@@ -826,6 +826,7 @@ try_.chromium_chromiumos_builder(
 
 try_.chromium_chromiumos_builder(
     name = "lacros-amd64-generic-rel",
+    branch_selector = branches.STANDARD_MILESTONE,
     builderless = not settings.is_main,
     main_list_view = "try",
     tryjob = try_.job(),