[AIRFLOW-3992] 1-setup-env.sh should be re-runable

[milton0825]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-3992
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.
  • In case you are proposing a fundamental code change, you need to create an Airflow Improvement Proposal(AIP).

Description

• Here are some details about my PR, including screenshots of any UI changes:
  We may rerun /app/scripts/ci/run-ci.sh to launch tests multiple times during development. Making it ln -f so that it does not fail with:

+ ln -s /home/airflow/.ssh/authorized_keys /home/airflow/.ssh/authorized_keys2
ln: failed to create symbolic link '/home/airflow/.ssh/authorized_keys2': File exists
ERROR: InvocationError for command '/app/scripts/ci/1-setup-env.sh' (exited with code 1)
______________________________________________ summary _______________________________________________
ERROR:   py27-backend_sqlite-env_docker: commands failed

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
  • All the public functions and the classes in the PR contain docstrings that explain what it does

Code Quality

• Passes flake8

[feng-tao]

@milton0825 LGTM

[codecov-io]

Codecov Report

Merging #4817 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4817   +/-   ##
=======================================
  Coverage   74.49%   74.49%           
=======================================
  Files         450      450           
  Lines       28997    28997           
=======================================
  Hits        21600    21600           
  Misses       7397     7397

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[ashb]

This is dangerous - it a user happens to run this on their host it will overwrite files!