Rename env to EC2_INSTANCE_TAG

Proposed changes

This PR fixes an issue where the ec2-stop-by-tag experiment fails to target the intended EC2 instance.

The ec2-stop-by-tag experiment encounters a TARGET_SELECTION_ERROR due to an incorrect retrieval of the EC2_INSTANCE_TAG environment variable.

Fault Summary:
TARGET_SELECTION_ERROR
{"errorCode":"TARGET_SELECTION_ERROR","phase":"PreChaos","reason":"failed to get the instance tag, invalid instance tag","target":"{EC2 Instance Tag: , Region: ap-northeast-2}"}

By correctly renaming the environment variable from INSTANCE_TAG to EC2_INSTANCE_TAG, we can accurately target the intended EC2 instance.

Types of changes

What types of changes does your code introduce to Litmus? Put an x in the boxes that apply

[ ] New feature (non-breaking change which adds functionality)
[x] Bugfix (non-breaking change which fixes an issue)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Documentation Update (if none of the other choices applies)

Checklist

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[x] I have read the CONTRIBUTING doc
[x] I have signed the commit for DCO to be passed.
[x] Lint and unit tests pass locally with my changes
[ ] I have added tests that prove my fix is effective or that my feature works (if appropriate)
[ ] I have added necessary documentation (if appropriate)

Special notes for your reviewer:

cc. @namkyu1999

litmuschaos / litmus