[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