HPCC-1960 Standalone diskwrite makes bad filename

Diskwrite activity in standalone mode incorrectly builds target filespec
if the specified file does not already exist. This fix ensures the correct
filespec is created in either case

Signed-off-by: William Whitehead <william.whitehead@lexisnexis.com>
Merge pull request #3901 from RussWhitehead/standAlone

HPCC-8684 When trying to OUTPUT a file in a standalone I get an error

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>