Use an always true BotDetector in fuchsia_asset_builder.dart

[zanderso]

The fuchsia_asset_builder tool runs during the Fuchsia build, and should never not be considered a bot. Analytics is already disabled for the entrypoint, but bot detection runs beforehand. Running the real bot detector is likely resulting in stray files in the Fuchsia source tree. This can be worked around by injecting a bot detector that always returns true here:

flutter/packages/flutter_tools/bin/fuchsia_asset_builder.dart

Line 33 in 3ec6978

Usage: () => DisabledUsage(),

Example hardwired bot detector:

flutter/packages/flutter_tools/test/src/mocks.dart

Line 702 in e216eec

class AlwaysTrueBotDetector implements BotDetector {

/cc @iskakaushik @jonahwilliams