[OSSCheck] Cache repos (#4456) (#4477)
[OSSCheck] Cache repos (#4456) (#4477)

This should speed up OSSCheck runs considerably.

In my local testing, cloning repos goes from 77s to 29s on a 100 Mbps
connection.
realm/SwiftLint,realm/SwiftLint,realm/SwiftLint,realm/SwiftLint