CMake: Use `list(APPEND x ...)` over `set(x ${x} ...)`.
CMake: Use `list(APPEND x ...)` over `set(x ${x} ...)`.

This removes a CMake anti-pattern where `set` was used to append to a
variable, instead of the more explicit `list(APPEND)` or
`string(APPEND)` alternatives.

Review: https://reviews.apache.org/r/59156/

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