fix: Fork PRs failing CocoaPods integration test

[calvincestari]

Part of #2563

• This PR changes the Podfile for the pod-install-project test project to build the pod location from CircleCI environment variables based on whether it is a forked PR or not.
• There is also an update to the documentation to inform users that when installing Apollo iOS with CocoaPods from a local development path, the CLI will not be automatically built for them.

There are three PRs to verify the change works as expected in each case:

1. PRs on the official repo - apollographql/apollo-ios

• These PRs use a branch as the pod source - Podfile line 12.
• PR Regular PR - pod-install-test #2714 is an example, which results in pod 'Apollo', :git => 'https://github.com/apollographql/apollo-ios.git', :branch => 'test/ci-env-dump'.
• CI builds of that PR can be found here.
• Looking at the CocoaPods Integration Tests job on the last build of that PR, for commit 6e34253, you can see the environment variables that are used in the "Preparing environment variables" step, and the successful "CocoaPods - Install" step.

2. PRs from forked repos with the same repo name - calvincestari/apollo-ios

• These PRs use a combination of the PR username/reponame and commit hash as the pod source - Podfile line 10.
• PR Forked PR - pod-install-test #2713 is an example, which results in pod 'Apollo', :git => "https://github.com/calvincestari/apollo-ios.git", :commit => '48f33050a57d2b5cd07a820bc310add2e272ac73'.
• CI builds of that PR can be found here.
• Looking at the CocoaPods Integration Tests job on the last build of that PR, for commit 48f3305, you can see the environment variables that are used in the "Preparing environment variables" step, and the successful "CocoaPods - Install" step.

3. PRs from forked repos but with a different repo name - calvincestari/custom-repo-name

• These PRs use a combination of the PR username/reponame and commit hash as the pod source - Podfile line 10.
• PR Custom Repo Forked PR - pod-install-test #2712 is an example, which results in pod 'Apollo', :git => "https://github.com/calvincestari/apollo-ios.git", :commit => '53ed7befddde11085243671c07c952346b000d3f'.
• CI builds of that PR can be found here.
• Looking at the CocoaPods Integration Tests job on the last build of that PR, for commit 53ed7be, you can see the environment variables that are used in the "Preparing environment variables" step, and the successful "CocoaPods - Install" step.

There is also the fact that this PR is from my fork of the repo and it has a successful CI pass.

[netlify]

👷 Deploy request for apollo-ios-docs pending review.

Visit the deploys page to approve it

Name	Link
🔨 Latest commit	66f6622

[AnthonyMDev]

LGTM!