Switch ci.yaml Linux builders to Debian. (#102253)

In preparation for upgrading linux VMs to Ubuntu, specify builders
to use 'os: Debian' instead of 'os: Linux'.

There should be no functional changes to current builders with this change.

Bug:102252

.ci.yaml

@@ -26,7 +26,7 @@ platform_properties:
         [
           {"dependency": "curl"}
         ]
-      os: Linux
+      os: Debian
       device_type: none
   linux_android:
     properties: