fix(ratelimiter): prevent REDIS_URL being overwritten if set

[ryanhristovski]

What type of PR is this?
fix(ratelimiter): prevent REDIS_URL from being overwritten if already set

What this PR does / why we need it:
This PR updates the logic to ensure that the REDIS_URL environment variable is only set if it isn’t already defined in the environment. This is especially important for when this issue is solved and proper Redis authentication is setup.

Example: I want to be able to set the REDIS_URL to a secret, which isn't possible in a configmap.

gateway-helm:
  config:
    envoyGateway:
      gateway:
        controllerName: gateway.envoyproxy.io/gatewayclass-controller
      provider:
        type: Kubernetes
        kubernetes:
          rateLimitDeployment:
            replicas: 1
            container:
              image: envoyproxy/ratelimit:master
            patch:
              type: StrategicMerge
              value:
                spec:
                  template:
                    spec:
                      containers:
                      - name: envoy-ratelimit
                        env:
                          - name: LOG_LEVEL
                            value: debug
                          - name: REDIS_URL
                            valueFrom:
                              secretKeyRef:
                                name: ratelimit-secret
                                key: redis_url
      rateLimit:
        backend:
          type: Redis

As of currently, not setting the url while having rateLimit set is putting Gateway in a crashloop.

Which issue(s) this PR fixes:
Fixes #

Release Notes: No

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.62%. Comparing base (5068698) to head (6d55586).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4722      +/-   ##
==========================================
+ Coverage   65.60%   65.62%   +0.01%     
==========================================
  Files         211      211              
  Lines       31995    31996       +1     
==========================================
+ Hits        20989    20996       +7     
+ Misses       9764     9761       -3     
+ Partials     1242     1239       -3     

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