Build the Linux AppImage for ARM64 (aarch64) alongside x86_64: the Linux CI
job now matrixes over both architectures, with arch-aware deps caching and
artifact/asset names (amd64 keeps its existing names). The aarch64 AppImage is
published to the nightly and release pages like the x86_64 one.
Run the unit-test suite on the aarch64 runner (faster GitHub arm runner); the
tests are built on that leg. Self-hosted keeps tests on the amd64 server.