bug.yml: fix missing label property

[carlocab]

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Submitting bug reports hasn't worked for a while because of the missing
label.

My editor complained about labels needing to be an array, so I've made
that one too. It was also complaining about the missing label property
for the checkboxes, so I guess it knows what it's talking about here.

See Homebrew/discussions#2526 (comment)

[BrewTestBot]

Review period skipped due to critical label.