[ci] disable flaky mobile-lightweight-dispatch-build

As title, fixing it is tracked in pytorch#78540. [skip ci] Pull Request resolved: pytorch#78557 Approved by: https://github.com/malfet, https://github.com/janeyx99

Author: suo

.github/workflows/pull.yml

@@ -275,6 +275,7 @@ jobs:
  docker-image-name: pytorch-linux-xenial-py3-clang5-android-ndk-r19c
 
  linux-focal-py3_7-gcc7-mobile-lightweight-dispatch-build:
+ if: false() # https://github.com/pytorch/pytorch/issues/78540
  name: linux-focal-py3.7-gcc7-mobile-lightweight-dispatch-build
  uses: ./.github/workflows/_linux-build.yml
  with: