Stop explicitly catching missing env-var error in composer

[jeffwidman]

This removes explicitly catching errors about missing env vars.

This is likely a bit controversial, so here's my thinking:

1. Dependabot at GitHub doesn't support injecting custom env vars. And for security reasons is unlikey to anytime soon: Dependabot environment variables #4660
2. The primary use case was for ACF Pro, but that is no longer relevant since they now support private composer auth natively: Support using custom PHP environment variables to pass API key to paid plugins #2001 (comment)
3. In general we've told users that if they need custom env vars for authentication reasons, they should instead ask their upstream providers to use the standard composer tooling, rather than a workaround. We aren't going to go out of our way to support non-standard tooling practices.
4. Setting env vars is still relevant for anyone who runs Dependabot standalone, but those are typically power users who are very comfortable debugging / reading the logs to realize the error is the lack of an env var.
5. Technically this could cause more internal alerts because removing this error changes those updates from being seen as "well-known user errors" to "unknown service errors" (which we internally alert on). However, I strongly suspect that very few users actually hit this problem... so if just a couple of repos are hitting it, that shouldn't be unduly affecting our metrics.
6. Currently unknown composer errors are hidden, but we should resolve that with a systemic solution rather than one-off-solutions for specific packages, which turns into a game of whack-a-mole: Expose composer error message when not resolvable #6290
7. Composer error handling is a big mess in our code... it's copy/pasted several places, with TODO notes saying we should refactor/cleanup our error handling code. And we don't even know what's still relevant once we drop composer 1 as the error messages changed a bit. So any cleanup / simplification we can do of the error handling code makes our future refactoring easier.

If we needed this generic error for other ecosystems, that'd make sense, but so far it's composer specific so until we need that, let's just remove it.

[deivid-rodriguez]

Turns out I need this error for other ecosystems. For example, this repo includes a .yarnrc.yml file that uses an environment variable, and that prevents dependabot from running. This user error is perfect for explaining this, so I'm closing this and will be opening a PR to catch this Yarn error and raise it as Dependabot::MissingEnvironmentVariable.

Let me know if you disagree though!

[jeffwidman]

Makes sense to me to keep it. 👍

[deivid-rodriguez]

#8446 will start using this error class 👍.