Set LUCI_CI to true to ensure docs are generated correctly

Bug: https://github.com/flutter/flutter/issues/117215
Change-Id: Iec0cb84e76b6068fc85a7ab4a988d2aee466c654
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/37420
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Drew Roen <drewroen@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>
(cherry picked from commit d308d276ce677a60f8323183b39e75759207cb44)
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/37502
Reviewed-by: Xilai Zhang <xilaizhang@google.com>

recipe_modules/adhoc_validation/api.py

diff --git a/recipe_modules/adhoc_validation/api.py b/recipe_modules/adhoc_validation/api.py
index 4c8e70f..275a09c 100644
--- a/recipe_modules/adhoc_validation/api.py
+++ b/recipe_modules/adhoc_validation/api.py
@@ -64,6 +64,7 @@
         checkout_path = self.m.repo_util.sdk_checkout_path()
         if (validation == 'docs') and self.m.repo_util.is_release_candidate_branch(checkout_path):
           env['LUCI_BRANCH'] = 'stable'
+          env['LUCI_CI'] = True
 
         with self.m.context(env=env, env_prefixes=env_prefixes):
           self.m.test_utils.run_test(validation, [resource_name])