Unrecognized option `P` for grep call in IdentityModel script

[mthalman]

The changes from dotnet/source-build-externals#183 didn't completely resolve the issues of being able to run the script amongst the various distros. Alpine is failing with the error grep: unrecognized option: P. That option is being used in the various grep calls that were added in that PR: example.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.