Fix github actions - Githubissues

On the latest colab image, the viewer cell in getting started part 3 notebook, does not seem to receive a signal that the cell was run, at least in non-interactive mode by papermill. That's why it was getting stuck at that step. Not sure how it was running okay in the previous colab image. So I modified the preprocessing script to comment out the viewer cells.

In addition, I made several other changes

[x] Edit .gitignore to ignore service account key files downloaded during the workflow
[x] The gha now gets triggered more meaningfully i.e only when the gha itself is edited or when the getting started notebooks are updated
[x] Added a safeguard to NOT authenticate gcloud if it is a pull request, as a conservative measure. I believe but did not dare to test that malicious pull requests may add a step that can expose the service account key. This however compromises our ability to test notebooks from a PR and I think that's okay
[x] Remove the outputs folder in the test folder as they do not have any relevance anymore
[x] Added a pull request step to update the csv file containing the list of colab images with sha256 digests. This is used to determine whether to pull the image from gcp or dockerhub

As found in the GitHub analytics repo, if a repo is inactive for 60 days, gha is disabled, but gh will send a notification to reenable the gha. However, it relies on admins and there is a legitimate chance that the notification email goes unnoticed. We need to decide if adding another step to create an empty pull request to update README.md (adding an extra space or empty line for example) 50 days after the last commit to automate this.

ImagingDataCommons / IDC-Tutorials

Fix github actions #47