Check for VIP_GO_APP_ENVIRONMENT before using it

[srtfisher]

Description

Ensure that VIP_GO_APP_ENVIRONMENT is defined before using it. Throws a fatal error when using in a CI environment where VIP_GO_APP_ENVIRONMENT is not defined but we are using https://github.com/Automattic/vip-go-mu-plugins/blob/develop/drop-ins/object-cache.php as our object-cache.php file.

Ensures that VIP_USE_NEXT_OBJECT_CACHE_DROPIN is still set to false for non-VIP_GO_APP_ENVIRONMENT environments.

Changelog Description

Pre-review checklist

Please make sure the items below have been covered before requesting a review:

• This change works and has been tested locally (or has an appropriate fallback).
• This change works and has been tested on a Go sandbox.
• This change has relevant unit tests (if applicable).
• This change uses a rollout method to ease with deployment (if applicable - especially for large scale actions that require writes).
• This change has relevant documentation additions / updates (if applicable).
• I've created a changelog description that aligns with the provided examples.

Pre-deploy checklist

• VIP staff: Ensure any alerts added/updated conform to internal standards (see internal documentation).

Steps to Test

[codecov]

Codecov Report

Merging #4889 (5437bf0) into develop (7e18e0a) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop    #4889   +/-   ##
==========================================
  Coverage      29.12%   29.12%           
  Complexity      4704     4704           
==========================================
  Files            276      276           
  Lines          20751    20751           
==========================================
  Hits            6044     6044           
  Misses         14707    14707