Remove -endpoint from src-cli usage

[efritz]

The endpoint flag has been deprecated and we should update all docker containers, github actions, and tools that depend on this flag to use the SRC_ENDPOINT envvar instead.

[efritz]

Deprecated (but not removed) in sourcegraph/src-cli#235. cc @ryanslade

[marekweb]

Dear all,

This is your release captain speaking. 🚂🚂🚂

Branch cut for the 3.19 release is scheduled for tomorrow.

Is this issue / PR going to make it in time? Please change the milestone accordingly.
When in doubt, reach out!

Thank you