re-enable windows build_tests (flutter#139599)

These have all passed since being introduced >20 commits ago:

![Screenshot from 2023-12-05 14-37-41](https://github.com/flutter/flutter/assets/7856010/997f39a1-377c-4e85-a7a1-02bb7e687506)

.ci.yaml

@@ -4585,7 +4585,6 @@ targets:
       - .ci.yaml
 
   - name: Windows build_tests_1_7
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -4604,7 +4603,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_2_7
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -4623,7 +4621,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_3_7
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -4642,7 +4639,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_4_7
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -4661,7 +4657,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_5_7
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -4680,7 +4675,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_6_7
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -4699,7 +4693,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_7_7
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties: