build: ignore backup files

Prevent backup files from being accidentally automatically added
to the index.

PR-URL: #28865
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: Rich Trott <[email protected]>

Author: AdamMajer

.gitignore

@@ -131,3 +131,4 @@ _UpgradeReport_Files/
 *.pyc
 __pycache__
 .DS_Store
+*~