Fix KeyError: 'Private' when using --all flag (#481)

The repository dictionary uses lowercase "private" key. Use .get() with
the correct case to match the pattern used elsewhere in the codebase.

The bug only affects --all users since --security-advisories short-circuits
before the key access.

Author: Iamrodos

github_backup/github_backup.py

@@ -1814,7 +1814,7 @@ def backup_repositories(args, output_directory, repositories):
             if args.include_milestones or args.include_everything:
                 backup_milestones(args, repo_cwd, repository, repos_template)
 
-            if args.include_security_advisories or (args.include_everything and not repository["Private"]):
+            if args.include_security_advisories or (args.include_everything and not repository.get("private", False)):
                 backup_security_advisories(args, repo_cwd, repository, repos_template)
 
             if args.include_labels or args.include_everything: