quickstart changes for MacOS for Issue #497

[dtseiler]

• Change sed syntax to support both GNU and BSD (MacOS) sed
• Look at uname -s to determine which binaries for pgo and expenv to copy to $GOBIN

Checklist:

• Have you added an explanation of what your changes do and why you'd like them to be included?
• Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?

Type of Changes:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

What is the current behavior? (link to any open issues here)
See Issue #497

What is the new behavior (if this is a feature change)?
sed and binary issues fixed.

Other information:
None.

[dtseiler]

Thanks to Dave Youatt for the help in troubleshooting the problem and crafting a solution.

[jmccormick2001]

thanks Don, I'll ask Val C to test this via her Mac.