…(brentybh)

This PR was squashed before being merged into the 4.3 branch (closes #33357).

Discussion
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[FrameworkBundle] Fix about command not showing .env vars

Before this fix, `bin/console about` can't properly show `Environment (.env)` section because:
`SYMFONY_DOTENV_VARS` which stores all keys of Dotenv-set env vars, is being fetched via `getenv()`.

However, in Symfony Dotenv 4.3, usage of `putenv()` is deprecated:
symfony/dotenv@d2fa94d#diff-a6967492da82dce9ba93bcba3eee0334
so we can get env vars via `getenv()` only when `new Dotenv(true)`.
In the default shipped `config/bootstrap.php`, there is `new Dotenv(false)` set.

(Maybe related #29131)

| Q             | A
| ------------- | ---
| Branch?       | 4.3 for bug fixes <!-- see below -->
| Bug fix?      | yes
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| License       | MIT

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Commits
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f48f19d [FrameworkBundle] Fix about command not showing .env vars