If `wp-admin/admin.php` has CRLF line endings, running `wp
--context=admin` with any command results in a PHP fatal error because
the regular expression to strip the wp-load.php require statement

Fixes wp-cli#5844
Props @willrowe for the regex fix.

xknown deleted the fix/context-admin-php-fatal branch

xknown restored the fix/context-admin-php-fatal branch