CircleCI jobs are run for any PR that is submitted to the repo. This PR adds a check to decide whether the job should be run or not.
Figured out that CircleCI has a way to gracefully terminate a job:
```
circleci-agent step halt
```
A `.ciignore` file is ran against all the changes in the PR to decide whether the PR should be built or not. If the answer comes out as `no`, a file is written at `/buid/skip_job.txt`. This is done in the `check_build_worthiness` step.
All further jobs, in the beginning, looks for this file and gracefully terminates the job if this file is present. The directory is passed down to the jobs as the workspace.
```yaml
skip_job_on_ciignore: &skip_job_on_ciignore
run: |
if [ -f /build/skip_job.txt ]; then
echo "halting job due to /build/skip_job.txt"
circleci-agent step halt
fi
```
ref: https://support.circleci.com/hc/en-us/articles/360015562253-Conditionally-end-a-running-job-gracefully
There are some known issues on jobs that are run when PR is merged to master, need to address them after this PR is merged.
