Fix device_os requested in linux_build_test tests (flutter#152808)

The MotoG4's had device_os=N, the mokey's have device_os=U.

Eventually I'll get this right...

Author: zanderso

.ci.yaml

@@ -2855,7 +2855,7 @@ targets:
       task_name: flutter_gallery__transition_perf
       artifact: gallery__transition_perf
       drone_dimensions: >
-        ["device_os=N","os=Linux", "device_type=mokey"]
+        ["device_os=U","os=Linux", "device_type=mokey"]
 
   - name: Linux_build_test flutter_gallery__transition_perf_e2e
     recipe: devicelab/devicelab_drone_build_test
@@ -2868,7 +2868,7 @@ targets:
       task_name: flutter_gallery__transition_perf_e2e
       artifact: gallery__transition_perf_e2e
       drone_dimensions: >
-        ["device_os=N","os=Linux", "device_type=mokey"]
+        ["device_os=U","os=Linux", "device_type=mokey"]
 
   - name: Linux_build_test flutter_gallery__transition_perf_hybrid
     recipe: devicelab/devicelab_drone_build_test
@@ -2881,7 +2881,7 @@ targets:
       task_name: flutter_gallery__transition_perf_hybrid
       artifact: gallery__transition_perf_hybrid
       drone_dimensions: >
-        ["device_os=N","os=Linux", "device_type=mokey"]
+        ["device_os=U","os=Linux", "device_type=mokey"]
 
   # linux mokey benchmark
   - name: Linux_mokey flutter_gallery__transition_perf_with_semantics