Updates scripts to use environment variables for the broker host

What does this MR do and why?

With Pact Broker being hosted on a Compute Engine instance, we now have a shared instance we can use to use Pact. This MR updates the URLs so it's not hardcoded to localhost and takes an environment variable to determine where to upload the contract and where the contract to verify is located.

Related to #370405 (closed)

How to set up and validate locally

• To publish contracts, run QA_PACT_BROKER_HOST="xx" PACT_BROKER=true bundle exec rake xx
  • The list of rake tasks for contract tests can be found by running bundle exec rake -T contracts
• To verify the contracts, run QA_PACT_BROKER_HOST="xx" sh spec/contracts/publish-contracts.sh "all"

Note: The url can be found in #370405 (comment 1304778651). It is an internal note as I would prefer this test instance not to be public so also be mindful with referencing this url in comments 🙃

MR acceptance checklist

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