fix bash math for pr-sanity-check? (#100735)
The current version fails with `.github/scripts/pr-sanity-check.sh: line 44: "0" + "5": syntax error: operand expected (error token is ""0" + "5"")`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100735
Approved by: https://github.com/cpuhrsch, https://github.com/malfet
