Fix false-positive ImpureVariable issue

[ghostwriter]

This patch resolves #10974, via the following changes:

• Add test case
• Update VariableFetchAnalyzer.php

[VincentLanglet]

Any idea why you get Could not find a usable config.yml, you may have revoked the CircleCI OAuth app. error @ghostwriter ? Is it fixed with some rebase ?

@orklah @weirdan Any time to review this bugfix ? Thanks a lot ! :)

[ghostwriter]

Any idea why you get Could not find a usable config.yml, you may have revoked the CircleCI OAuth app. error @ghostwriter ? Is it fixed with some rebase ?

No rebase is needed. The maintainers just need to approve the GitHub Actions workflows for this pull request. (GitHub security feature.)

"5 workflows awaiting approval"

[VincentLanglet]

There is this error

**CircleCI Pipeline ** — Could not find a usable config.yml, you may have revoked the CircleCI OAuth app.

[ghostwriter]

There is this error

**CircleCI Pipeline ** — Could not find a usable config.yml, you may have revoked the CircleCI OAuth app.

Correct, because the workflows are not approved to be triggered by my pull request. (GitHub is not generating the GITHUB_TOKEN for CircleCI)

When a user pushes a commit, opens a PR, or triggers a new CircleCI build via the API or via the UI, CircleCI fetches the configuration file (config.yml) to generate the pipeline. CircleCI will use the author's authentication token (Automatically generated by GitHub per PR) to fetch the config.yml file. When this authentication fails because the token has become invalid or has expired, the user will see the above error message.