Case sensitivity for environment names

## Problem
I've come across an issue in my use of github/lock to lock and unlock environments.

In our workflows, we occasionally used mixed cases when specifying environments (`STAGING` and `staging` - work in progress to standardize these). GitHub itself is case insensitive when using the environments natively, and won't let you create an upper-case version of an existing environment because its a duplicate:


However, this action appears to be **case sensitive** (probably due to branches themselves being case sensitive?).

## Ask
It would be really nice if the action compared the input environment with the lock environment without caring about the case of either, matching how environments work otherwise. So I could input STAGING and if my lock branch is `staging-branch-deploy-lock`, it will come back as locked. The current behavior does not see the lock:


Start by locating the environment-to-lock comparison in the JavaScript GitHub Action; the issue names no files or tests. Check whether mixed-case environment names are covered, then verify that equivalent casing matches the existing lock behavior without changing unrelated branch handling.

索引モデルが issue の本文から書いたものです。