fix(latexindent): regression in f2063898e9

[Tejada-Omar]

When ran with the -l flag and no other filename (as is the case when passing the file through STDIN), latexindent interprets the argument to -l as the file to be formatted

Eg. latexindent -l=foo.yaml < bar.tex overwrites the current buffer with foo.yaml rather than the formatted bar.tex

This commit just appends the argument - so that STDIN is always used

See: f2063898e943893b20aeb30a324d5364ebaddff6 (#654)

Note: Issue can be replicated by passing the following option to setup:

formatters = {
	latexindent = {
		prepend_args = { "-l=foo.yaml" },
	},
},

The formatter will overwrite the buffer with the attempted formatting of foo.yaml if it exists, and just error out if it doesn't