Cylc lint: U999 is over-eager

[ColemanTom]

I found U999 was complaining about something it should not be complaining about. Take this snippet.

$ cat flow.cylc
    [[xtriggers]
        foo = abc(a=123, host=hello)
        foo = abc(a=123, \
            host=hello \
        )
$ CYLC_VERSION=8.4 cylc lint $PWD
[U999] flow.cylc:4: [runtime][__MANY__][workflow state polling]host - not available at Cylc 8

Checked /g/ns/sc/user-cylc/tcoleman/code/basic-workflow against ['728', 'style'] rules and found 1 issue.
$ CYLC_VERSION=8.4 cylc version
8.4.1

When host is on a new line, it throws an error, thinking it belongs to workflow state polling, even though it is part of an xtrigger and host is a completely valid and reasonable kwarg.

[wxtim]

This is going to be tricky to solve, becuase U999 is auto generated: It's not a simple matter of modifying a rule.