Add Environment Variable options to flags managed in exporter-toolkit

[oseoin]

Proposed changes

Adding environment variables in exporter-toolkit directly was rejected, so adding here.

LISTEN_ADDRESS=:9114 go run .                
ts=2024-01-16T09:52:54.647Z caller=exporter.go:136 level=info msg="Starting nginx-prometheus-exporter" version="(version=, branch=, revision=unknown)"
ts=2024-01-16T09:52:54.647Z caller=exporter.go:137 level=info msg="Build context" build_context="(go=go1.21.4, platform=darwin/arm64, user=, date=, tags=unknown)"
ts=2024-01-16T09:52:54.648Z caller=tls_config.go:313 level=info msg="Listening on" address=[::]:9114

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING
  guide
• I have proven my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have ensured the README is up to date
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch on my own fork

[jjngx]

👍🏻