Fix finding grep.exe path on Windows

[rok-cesnovar]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

In the edge case where the users do not have grep.exe in their system path, we need to wrap the Sys.which() call in the with_path call just like we do the rest of the system calls.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Rok Češnovar

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[rok-cesnovar]

The 4.2 Windows tests are failing for other reasons (see #660) so this should be good to go.