Don't allow try-builds to be rolled up

[compiler-errors]

Pending try builds have their checkboxes unchecked, even though the code suggests they should be disabled:

https://github.com/rust-lang/homu/blob/c8f51b8e261541c13747e50097cd8899aae9537b/homu/html/queue.html#L180-L184

I can see someone accidentally rolling up a try build because of this :cold_sweat:

[compiler-errors]

state.status == 'pending' and not state.try_ should be False :'(