…n GitHub AE, and check it first.

Mitigation for OOM errors (137/SIGKILL) seen by users when we overcommit the available memory.
For Unix, reserve 1GB.
For Windows, reserve 1.5GB, as the OS needs more memory and estimates inaccurately.

Specifying a token as a cli input leads to a potential for leaking the
token on CI logs. This commit removes the option. Instead, users
should specify a single GitHub token through `--github-auth-stdin` or
by setting the `GITHUB_TOKEN` environment variable. This token should be
created with enough privileges to access the required repository.

This commit adds two new ways of specifying GitHub auth:

1. from the GITHUB_TOKEN environment variable
2. from standard input

This commit does not include any documentation changes and the
descriptions of new command line options will need to be tweaked.

Retrieve the relevant values in the method itself instead.