cros-bisector: ansible: force switch to user cros-bisect-service

Use become_user to force switch user instead.

BUG=b:165128081
TEST=ansible-playbook main.yml --check
TEST=ansible-playbook main.yml

Change-Id: I852ebf940abb1cd062a33483e5e5901e42f68cad
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/cros-bisector/+/2366954
Tested-by: Zheng-Jie Chang <zjchang@chromium.org>
Auto-Submit: Zheng-Jie Chang <zjchang@chromium.org>
Commit-Queue: Kuang-che Wu <kcwu@chromium.org>
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
diff --git a/ansible/main.yml b/ansible/main.yml
index 14960c6..4b0f92d 100644
--- a/ansible/main.yml
+++ b/ansible/main.yml
@@ -3,7 +3,8 @@
 # found in the LICENSE file.
 
 - hosts: localhost
-  user: cros-bisect-service
+  become: yes
+  become_user: cros-bisect-service
   roles:
     - setup-debian
     - setup-iam