pr-upload: tighten json_files glob

[dawidd6]

Tighten the glob, so we won't accidentaly parse arbitrary JSON files and only valid bottle ones.

I bumped into an issue in my tap today where tap_migrations.json was read first.

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[BrewTestBot]

Review period will end on 2020-12-15 at 00:00:00 UTC.

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Good catch!