The GitHub app e2e test requires a GitHub app to be created and
installed, and also requires a few environment variables to be set.
This commit updates the GitHub action workflow by providing the
environment variables which can be set via GitHub Secret. GitHub
Secrests cannot start with `GITHUB_`. Hence, this commit prepends
`TEST_` to the env variables.
It also updates how GitHub app private key file is set. It can be set by
either `TEST_GITHUB_APP_PRIVATE_KEY` or
`TEST_GITHUB_APP_PRIVATE_KEY_FILE`.
Nan Yu
Liam Wyllie