Mock skip_pull in specs to prevent failures when ENV is set

What does this MR do and why?

Some specs fail if a user has QA_SKIP_PULL set on their own machine.
We should avoid having our specs depend on the environment variables set (or not set) on a specific machine, and instead configure the tests to have the expected state.

How to set up and validate locally

# both of these should pass with this change, confirming that external environmental state doesn't impact tests
QA_SKIP_PULL=true bundle exec rspec
QA_SKIP_PULL=false bundle exec rspec

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.