feat: allow configFileNames to be set through env

[scootermon]

Changes

This PR changes the configFileNames config option (introduced recently in #36644) to support setting it through environment variables as well.
There is a chance I'm missing an obvious reason why it was done this way, but neither the PR nor the corresponding issue mentions one.

Context

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• This closes an existing Issue, Closes: #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

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The public repository:

[CLAassistant]

All committers have signed the CLA.

[RahulGautamSingh]

Please, also test this on a test repo, to be sure.

[scootermon]

Please, also test this on a test repo, to be sure.

See: https://github.com/scootermon/renovate-test-39144

The onboarding PR is using the correct filename: scootermon/renovate-test-39144#1

and a subsequent run managed to find the file:

DEBUG: Found my-cool-renovate.json5 config file (repository=scootermon/renovate-test-39144)
DEBUG: Repository config (repository=scootermon/renovate-test-39144)
       "fileName": "my-cool-renovate.json5",
       "config": {"$schema": "https://docs.renovatebot.com/renovate-schema.json"}

[jamietanna]

Discussing internally, one potential risk here is that this may lead to API calls increasing for a self-hosted administrator

However, I think it's reasonable to be able to move this from config.js to environment variables/CLI options, given it's already available in the config.js 👍

[github-actions]

🎉 This PR is included in version 42.13.0 🎉

The release is available on:

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• 42.13.0

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