fix: enable set positional-arguments in justfile

[bolinfest]

The way these definitions worked before, they did not handle quoted args with spaces properly.

For example, if you had /tmp/test-just/printlen.py as:

#!/usr/bin/env python3

import sys

print(len(sys.argv))

and your justfile was:

printlen *args:
    /tmp/test-just/printlen.py {{args}}

Then:

$ just printlen foo bar
3
$ just printlen 'foo bar'
3

which is not what we want: 'foo bar' should be treated as one argument.

The fix is to use positional-arguments:

set positional-arguments

printlen *args:
    /tmp/test-just/printlen.py "$@"