[owners] Update startup script with (redacted) current startup script and reorganize (#537)
[owners] Update startup script with (redacted) current startup script and reorganize (#537)

* Update startup script with (redacted) current startup script

* Add comments indicating where env vars are used

* Organize env var declarations

* Drop unused environment variables

* Replace PRIVATE_KEY_PATH with PRIVATE_KEY

* Add lots of comments describing sections of startup script

ampproject/amp-github-apps,ampproject/amp-github-apps,ampproject/amp-github-apps,ampproject/amp-github-apps,ampproject/amp-github-apps