Make runps-file to use runps-simple & script.ps1

Works much better. Removes dependency on `--file` switch in our
executable (which is not a PowerShell thing) and instead uses the
canonical method of launching a script/executable in PowerShell (the
ampersand). Coincidentally, this now makes the output not depend on a
TTY, and so `monad-run make hashbang-tests` now works as expected.

scripts/runps-file.sh

@@ -2,4 +2,4 @@
 
 SCRIPTDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
 
-"$SCRIPTDIR/runps.sh" --file "$@"
+"$SCRIPTDIR/runps-simple.sh" "& $@"