Fix CI issues for forked repos

[imjasonh]

This fixes two CI-related issues hit while proposing #536

1. e2e tests are run for all pushes, even to forked repos. This causes them to fail when they don't have the required secret configured. See https://github.com/imjasonh/cosign/actions/runs/1125396335 -- this change disables e2e tests when the repo != sigstore/cosign, and tells GitHub Actions to run the workflow when it's as a PR to the cosign repo, not only when it's a push. Also allow manual execution, because why not.
2. CI tests failed due to an out-of-date module dependency. See https://github.com/imjasonh/cosign/actions/runs/1125396339 -- this change runs the go get command in that error message.

This change also removes some unnecessary comments from e2e_tests.yml for (I think) better readability.