Stop triggering E2E test in pull request.

[Kesin11]

Problem

Previously, I introduced E2E test to check xcode-install can install Xcode correctly. #365
But, recently this test failed all pull request. (for example #378 #382 )

I read failed test log and found env variable XCODE_INSTALL_USER and XCODE_INSTALL_PASSWORD maybe null.
https://github.com/xcpretty/xcode-install/blob/master/.github/workflows/e2e.yml#L15-L16

Github Actions seems does not exposed "secrets" for action trigger by pull requests that from forked repository.
https://github.community/t5/GitHub-Actions/Make-secrets-available-to-builds-of-forks/td-p/30678

The test for "#372" is successful, but that's because the admin @milch created the pull request directly in this repository

Suggestion

In my opinion, E2E test should be stopped until Github Actions provides a feature that is a solution to this problem.