Gracefully handle duplicate dotenv keys

What does this MR do and why?

If a dotenv artifact contained the same key, CI artifact uploads would fail with 400 Bad Request. For example:

KEY1=test
KEY1=something

In a shell environment, we'd expect the latest value to overwrite the first one, so we now de-duplicate the dotenv keys and use the latest value.

This is consistent with how the original dotenv package works. Given an .env file and JavaScript test:

KEY=one
KEY=two
KEY=three

require('dotenv').config()

console.log(process.env.KEY)

The result:

$ node test.js
three

Relates to #288064

How to set up and validate locally

Run a CI job with https://gitlab.com/jrreid/duplicate-dotenv-test/-/blob/main/.gitlab-ci.yml.

MR acceptance checklist

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