Migration from Bamboo to Github Actions for Nightly Builds

[Sebastian Echegaray]

Hello,

We have deprecated Bamboo as our build system. We have moved to using Github Actions to run our nightly builds. 

Sandbox releases can be found now here: https://github.com/cdapio/cdap-build/releases/

Nightly test results can be found: https://github.com/cdapio/cdap-build/actions

Please let me know if you have any questions,

-Sebastian