scripts: release: use GITHUB_TOKEN and start_date in scripts

Updated bug_bash.py and list_issues.py to use the GITHUB_TOKEN
environment variable for consistency with other scripts.

Updated bug_bash.py to use `-s / --start-date` instead of
`-b / --begin-date`.

Signed-off-by: Christopher Friedt <cfriedt@fb.com>
(cherry picked from commit 3b3fc278)