Return an error when additional headers conflicts

[jhchabran]

Passing SRC_ADDITIONAL_HEADERS_AUTHORIZATION=... and SRC_ACCESS_TOKEN at the same time leads to the latter overriding the former silently, making it quite difficult to spot.

This PR surfaces the conflict as an error explictly advising to never set SRC_ACCESS_TOKEN in conjunction with the additional header.

Test plan

Unit test + local run:

# OK:

~/work/src-cli/cmd  jh/fix-authorization-header-conflict $ SRC_HEADER_AUTHORIZATION="sourcegraph" SRC_ENDPOINT=https://sourcegraph.test:3443 go run ./... api -query='query { currentUser { username } }'
{
  "data": {
    "currentUser": {
      "username": "sourcegraph"
    }
  }
}

# Prints an error because that's doomed to fail: 

~/work/src-cli/cmd  jh/fix-authorization-header-conflict $ SRC_ACCESS_TOKEN="foobar" SRC_HEADER_AUTHORIZATION="sourcegraph" SRC_ENDPOINT=https://sourcegraph.test:3443 go run ./... api -query='query { currentUser { username } }'
reading config: when passing an 'Authorization' additional headers, SRC_ACCESS_TOKEN must never be set
exit status 1
~/work/src-cli/cmd  jh/fix-authorization-header-conflict $