[gh-actions] Move to actions/checkout@v2

[opacam]

To fix gh-actions failures on re-running checks, like the one we have at here for #2069

[AndreMiras]

It seems like we get a regression on the rebuild_updated_recipes.

ANDROID_SDK_HOME=/home/user/.android/android-sdk ANDROID_NDK_HOME=/home/user/.android/android-ndk \
venv/bin/python ci/rebuild_updated_recipes.py
Traceback (most recent call last):
  File "ci/rebuild_updated_recipes.py", line 112, in <module>
    main()
  File "ci/rebuild_updated_recipes.py", line 77, in main
    recipes = modified_recipes()
  File "ci/rebuild_updated_recipes.py", line 43, in modified_recipes
    git_diff = sh.contrib.git.diff('--name-only', branch)
  File "/home/user/app/venv/lib/python3.6/site-packages/sh.py", line 1427, in __call__
    return RunningCommand(cmd, call_args, stdin, stdout, stderr)
  File "/home/user/app/venv/lib/python3.6/site-packages/sh.py", line 774, in __init__
    self.wait()
  File "/home/user/app/venv/lib/python3.6/site-packages/sh.py", line 792, in wait
    self.handle_command_exit_code(exit_code)
  File "/home/user/app/venv/lib/python3.6/site-packages/sh.py", line 815, in handle_command_exit_code
    raise exc
sh.ErrorReturnCode_128: 

  RAN: /usr/bin/git diff --name-only origin/develop

[AndreMiras]

LGTM