It's sometimes necessary to have a `livecheck` block in an `on_macos`
or `on_linux` block. For example, a formula may be disabled on macOS
but not on Linux. In that scenario, we only want a `livecheck` block
to apply to Linux, so livecheck will automatically skip the formula
as disabled on macOS.
While this setup works on a technical level, `brew style` will give
an `on_linux cannot include livecheck` offense. This commit addresses
the issue by adding `livecheck` to `on_system_allowed_methods` in the
`ComponentsOrder` Rubocop.
This also updates `on_system_allowed_methods` to use the order in
`FORMULA_COMPONENT_PRECEDENCE_LIST`, which may make it a bit easier
for formula maintainers to read at a glance.
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