Add '$PHPBUILD_KEEP_OBJECT_FILES' environment variable not to remove object files in build directory
Add '$PHPBUILD_KEEP_OBJECT_FILES' environment variable not to remove
object files in build directory

rogeriopradoj/php-build,hnw/php-build,hirak/php-build,0livier/php-build,circleci/php-build,php-build/php-build,gerardroche/php-build