Rename env to EC2_INSTANCE_TAG

[jongwooo]

Proposed changes

Related issue #4807 (comment)

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.

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