The aarch64 integration script hardcoded `LAST_RELEASE_VERSION="v39.0"`
for the live-upgrade binary download, while the live-migration runner
already accepts a `MIGRATABLE_VERSION` env override with a `vxx.0`
regex check. Standardise the aarch64 script on the same env-override
block so both arches honour the same knob with the same validation.
Default is unchanged (v39.0).
Assisted-by: Claude:Opus-4.7
Signed-off-by: Rob Bradford <rbradford@meta.com>