Use AWK instead of sed to create BOM in feature test by mrsdizzie · Pull Request #5728

Use AWK instead of sed to create BOM in feature test by mrsdizzie · Pull Request #5728 · wp-cli/wp-cli

The current test fails on MacOS because of:

  `sed: -I or -i may not be used with stdin`

After fixing that,  I found sed on MacOS works in text mode vs binary mode and it is difficult have it make a binary change like this. Use AWK instead since it works the same everywhere in this case.

Fixes wp-cli#5723